Clarify that C modules should be optional.

HACKING

@@ -1,5 +1,5 @@
 Please follow PEP8 with regard to coding style.
 
-All functionality should be available in pure Python. C replacements may 
-be written for performance reasons, but should never replace the Python 
+All functionality should be available in pure Python. Optional C implementations
+may be written for performance reasons, but should never replace the Python 
 implementation. The C implementations should follow the kernel/git coding style.