README: add rules for writting pages

.github/README.md

@@ -1,5 +1,4 @@
-cheatsheets
-===========
+# cheatsheets #
 This repository contains community-sourced cheatsheets to be used with
 [cheat][] and similar applications.
 
@@ -22,10 +21,26 @@ git add --all
 git commit -m <message>
 ```
 
-As a guideline, it is preferred to use [docopt][] syntax when specifying
-parameter placeholders. In edge-cases where that syntax may cause confusion, it
-is permissible to use placeholder values (`foo.txt`, `example.com`, etc.) as
-necessary.
+To take a long story short here are our guidelines for cheatsheets:
+
+- If the cheatsheet contains frontmatter, it must be valid.
+- Cheatsheets should generally be scoped to a single CLI command (most often POSIX-compatible CLI command).
+  - It's not always possible. For instance pages describing shell syntax often can't avoid describing
+    multiple CLI commands as almost everything in traditional shells is a command.
+  - Don't create pages for CLI commands with a good help system like `fish`, etc because users of them already have
+    neat completion system/man pages written by other contributors and maintainers.
+- Cheatsheets should end with a newline, but not a blank line.
+- Code blocks should be separated by a single blank line.
+- The comment line should end with a colon: `# To foo the bar:`, rather than `# To foo the bar`.
+  Note for users coming from `TlDr` project: it's preferred to use `To` at the beggining of the sentence.
+- Text should be reduced near to the minimum necessary to do the job. Avoid commentary and context where possible.
+- Where possible, cheatsheets should be consistent both internally, and with other sheets.
+- Excluding cases where it might cause confusion, commands should generally conform to [docopt][] syntax.
+  It's better not write complex placeholders containing `<` or `>` inside them to demonstrate for instance how comparison
+  operators work in `[[`. In such cases the best option is just omit placeholders.
+  - For path placeholders use `<in-file>`, `<out-file>`, or `<file>`. Almost the same applies to directories
+    but instead of term `file` `dir` term is used.
+  - Number placeholders with the following format: `<placeholder-1>` eg. `<in-file-1>`.
 
 ### License ###
 Cheatsheets are licensed under [Creative Commons CC0 1.0][cc0]. See