Doc: EXTENSIONS.md, Cppcheck, INSTALLATION.md refactoring (#199)

- updated README.md;
- moved CONTRIBUTING.md to docs/;
- splitted INSTALLATION.md to POSIX.md, WINDOWS.md and HOW_TO_USE.md;
- added Extensions doc;
- added a mention of how Cppcheck can be used together with CMake;

---------
Co-authored-by: Paul <lucky@nzbget.com>

README.md

@@ -60,13 +60,18 @@ QNAP packages are available as native packages and buildroot packages. [QNAP rea
 
 [General instructions](INSTALLATION.md)
 
-[Linux](linux/build-info.md) [macOS](osx/build-info.md) [Synology](synology/build-info.md) [QNAP](qnap/build-info.md)
+## Extensions
+ - [V1 (NZBGet v22 and below)](docs/extensions/EXTENSIONS_LEGACY.md)
+ - [V2 (NZBGet v23 and above)](docs/extensions/EXTENSIONS.md)
+
+## Brief introduction on how to use NZBGet
+ - [How to use](docs/HOW_TO_USE.md)
 
 ## Contribution
 
 Contributions are very welcome - not only from developers, but from our users too - please don't hesitate to participate in [discussions](https://github.com/nzbgetcom/nzbget/discussions) or [create a new discussion](https://github.com/nzbgetcom/nzbget/discussions/new/choose) 
 
-For more information - see [Contributing](CONTRIBUTING.md).
+For more information - see [Contributing](docs/CONTRIBUTING.md).
 
 ## Donate
 

docs/HOW_TO_USE.md

@@ -0,0 +1,208 @@
+## Configuration
+
+NZBGet needs a configuration file. 
+
+An example configuration file is provided in "nzbget.conf", which
+is installed into "`<prefix>`/share/nzbget" (where `<prefix>` depends on
+system configuration and configure options - typically "/usr/local",
+"/usr" or "/opt"). The installer adjusts the file according to your
+system paths. If you have performed the installation step 
+"make install-conf" this file is already copied to "`<prefix>`/etc" and
+NZBGet finds it automatically. If you install the program manually
+from a binary archive you have to copy the file from "`<prefix>`/share/nzbget"
+to one of the locations listed below.
+
+Open the file in a text editor and modify it accodring to your needs.
+
+You need to set at least the option "MAINDIR" and one news server in
+configuration file. The file has comments on how to use each option.
+
+The program looks for configuration file in following standard 
+locations (in this order):
+
+On POSIX systems:
+```
+<EXE-DIR>/nzbget.conf
+~/.nzbget
+/etc/nzbget.conf
+/usr/etc/nzbget.conf
+/usr/local/etc/nzbget.conf
+/opt/etc/nzbget.conf
+~/usr/etc/nzbget.conf
+```
+
+On Windows:
+```
+<EXE-DIR>\nzbget.conf
+```
+
+If you put the configuration file in other place, you can use command-
+line switch "-c `<filename>`" to point the program to correct location.
+
+In special cases you can run program without configuration file using
+switch "-n". You need to use switch "-o" to pass required configuration 
+options via command-line.
+
+## Usage
+
+NZBGet can be used in either standalone mode which downloads a single file 
+or as a server which is able to queue up numerous download requests.
+
+TIP for Windows users: NZBGet is controlled via various command line
+parameters. For easier using there is a simple shell script included
+in "nzbget-shell.bat". Start this script from Windows Explorer and you will
+be running a command shell with PATH adjusted to find NZBGet executable.
+Then you can type all commands without full path to nzbget.exe.
+
+### Standalone mode:
+--------------------
+```
+nzbget <nzb-file>
+```
+### Server mode:
+----------------
+
+First start the nzbget-server:
+
+  - in console mode:
+```
+nzbget -s
+```
+  - or in daemon mode (POSIX only):
+```
+ nzbget -D
+```	
+  - or as a service (Windowx only, firstly install the service with command "nzbget -install"):
+```
+  net start NZBGet
+``` 
+To stop server use:
+```
+nzbget -Q  
+```
+TIP for POSIX users: with included script "nzbgetd" you can use standard
+commands to control daemon:
+```
+nzbgetd start
+nzbgetd stop
+```
+
+When NZBGet is started in console server mode it displays a message that
+it is ready to receive download requests. In daemon mode it doesn't print any
+messages to console since it runs in background.
+
+When the server is running it is possible to queue up downloads. This can be
+done either in terminal with "nzbget -A <nzb-file>" or by uploading
+a nzb-file into server's monitor-directory (<MAINDIR>/nzb by default).
+
+To check the status of server start client and connect it to server:
+```
+nzbget -C
+```
+The client have three different (display) outputmodes, which you can select
+in configuration file (on client computer) or in command line. Try them:
+```
+nzbget -o outputmode=log -C
+nzbget -o outputmode=color -C
+nzbget -o outputmode=curses -C
+```
+To list files in server's queue:
+```
+nzbget -L
+```
+It prints something like:
+```
+[1] nzbname\filename1.rar (50.00 MB)
+[2] nzbname\filename1.r01 (50.00 MB)
+[3] another-nzb\filename3.r01 (100.00 MB)
+[4] another-nzb\filename3.r02 (100.00 MB)
+```
+This is the list of individual files listed within nzb-file. To print
+the list of nzb-files (without content) add G-modifier to the list command:
+```
+[1] nzbname (4.56 GB)
+[2] another-nzb (4.20 GB)
+```
+The numbers in square braces are ID's of files or groups in queue.
+They can be used in edit-command. For example to move file with
+ID 2 to the top of queue:
+```
+nzbget -E T 2
+```  
+or to pause files with IDs from 10 to 20:
+```
+nzbget -E P 10-20
+```
+or to delete files from queue:
+```
+nzbget -E D 3 10-15 20-21 16
+```
+
+The edit-command has also a group-mode which affects all files from the
+same nzb-file. You need to pass an ID of the group. For example to delete
+the whole group 1:
+```
+nzbget -E G D 1
+```
+The switch "o" is useful to override options in configuration files. 
+For example:
+```
+nzbget -o reloadqueue=no -o dupecheck=no -o parcheck=yes -s
+```  
+or:
+```
+nzbget -o createlog=no -C
+```
+
+### Running client & server on seperate machines:
+-------------------------------------------------
+
+Since nzbget communicates via TCP/IP it's possible to have a server running on
+one computer and adding downloads via a client on another computer.
+
+Do this by setting the "ControlIP" option in the nzbget.conf file to point to the
+IP of the server (default is localhost which means client and server runs on 
+same computer)
+
+### Security warning
+--------------------
+
+NZBGet communicates via unsecured socket connections. This makes it vulnerable.
+Although server checks the password passed by client, this password is still 
+transmitted in unsecured way. For this reason it is highly recommended 
+to configure your Firewall to not expose the port used by NZBGet to WAN. 
+
+If you need to control server from WAN it is better to connect to server's
+terminal via SSH (POSIX) or remote desktop (Windows) and then run
+nzbget-client-commands in this terminal.
+
+### Web-interface
+-----------------
+
+NZBGet has a built-in web-server providing the access to the program
+functions via web-interface.
+
+To activate web-interface set the option "WebDir" to the path with
+web-interface files. If you install using "make install-conf" as
+described above the option is set automatically. If you install using
+binary files you should check if the option is set correctly.
+
+To access web-interface from your web-browser use the server address
+and port defined in NZBGet configuration file in options "ControlIP" and
+"ControlPort". For example:
+```
+http://localhost:6789/
+```
+
+For login credentials type username and the password defined by
+options "ControlUsername" (default `"nzbget"`) and "ControlPassword"
+(default `"tegbzn6789"`).
+
+In a case your browser forget credentials, to prevent typing them each
+time, there is a workaround - use URL in the form:
+```
+http://localhost:6789/username:password/
+```
+Please note, that in this case the password is saved in a bookmark or in
+browser history in plain text and is easy to find by persons having
+access to your computer.