updating docs and contribution steps

Author: caridy

CONTRIBUTING.md

@@ -20,6 +20,15 @@ To build files in `locale-data/` based on CLDR:
 
     grunt cldr
 
+
+Source Code
+-----------
+
+All the source code is in `src/` folder, written as ES6 modules, and transpiled
+using `es6-module-transpiler` into the `lib/` and `dist/` folders.
+
+The `dist/` is in git because of bower, make sure you commit those files as well.
+
 Release checklist
 -----------------