docs/develop/cmodules.rst: Add link to source code for user C example.

Signed-off-by: Damien George <damien@micropython.org>

docs/develop/cmodules.rst

@@ -61,8 +61,9 @@ Basic example
 
 This simple module named ``cexample`` provides a single function
 ``cexample.add_ints(a, b)`` which adds the two integer args together and returns
-the result. It can be found in the MicroPython source tree and has
-a source file and a Makefile fragment with content as descibed above::
+the result. It can be found in the MicroPython source tree
+`in the examples directory <https://github.com/micropython/micropython/tree/master/examples/usercmodule/cexample>`_
+and has a source file and a Makefile fragment with content as descibed above::
 
     micropython/
     └──examples/