readme update pointing contribs to nitrogen_core

README.markdown

@@ -9,6 +9,7 @@ technology stack.
 See the [Nitrogen Project website](http://nitrogenproject.com) for
 additional information.
 
+
 ## Getting Started
 
 Follow the tutorial at
@@ -34,6 +35,10 @@ Instructions for building on windows can be found in [rel/overlay/win](https://g
 
 ## Want to contribute?
 
+[nitrogen_core](https://github.com/nitrogen/nitrogen_core) is where you would
+find the vast majority of Nitrogen's codebase, should you wish to contribute or
+make changes.
+
 Read our [contribution
 guidelines](https://github.com/nitrogen/nitrogen/blob/master/CONTRIB.markdown)
 to get started contributing to Nitrogen!  (they're not strict, just