Merge pull request #595 from bovi/mrbgems-readme

Mention mrbgems in the README

README.md

@@ -59,6 +59,12 @@ To run the tests, execute the following from the project's root directory.
 
     $ make test
 
+## Customization
+
+mruby contains a package manager called *mrbgems*. To create extensions
+in C and/or Ruby you should create a *GEM*. You will find a complete
+documentation with examples under *doc/mrbgems*.
+
 ## License
 
 Copyright (c) 2012 mruby developers