Transform existing documentation for PORTING and USING to markdown

- Converted the existing documentation to markdown format.
- Fix for #1458.
- Changes font to bold at some places to improve readability

pkg/PORTING.md

@@ -0,0 +1,11 @@
+This directory provides some porting information for libraries and programs to
+use with RIOT (to build an external module). If you'd like to add a package to
+RIOT you need to add a directory with the name of your package to this directory.
+Your directory should contain at least two files:
+
+ * **patch.txt**- Your patch of the upstream application of the package to make
+ it build with RIOT.
+ * **Makefile**- A Makefile describing how to get the upstream application, apply
+ the patch and how to build the package as a RIOT module.
+ A rough template for several methods of acquiring a package
+ is provided in Makefile.git, Makefile.http, and Makefile.svn

pkg/USING.md

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+Packages are included to your application as external modules. Thus you only
+have to add the following line to your application (and update your INCLUDE path
+accordingly):
+
+    USEPKG += <pkg_name>