add rule about committing source code only

DEVELOPER.RULES

@@ -27,6 +27,24 @@ rules below, and you'll do ok with this multi-project project:
   it in mind that the copyright file is in a machine-readable
   standards-compliant format)
 
+* please ONLY ADD SOURCE CODE and ONLY ADD PYJAMAS-RELATED source code.
+
+  on no account add the source code of an external project to pyjamas:
+  create a download script which fetches and unpacks a stable revision
+  of that code.
+
+  on ABSOLUTELY NO ACCOUNT add any binary-object files, executables,
+  fonts, external images that were written by and for other projects,
+  or anything OTHER than pyjamas source code.
+
+  also please remember that the javascript that is auto-generated by
+  the pyjamas compiler is also considered to be "object code", and as
+  such should NEVER be added to the pyjamas repository.
+
+  anything that is needed must either be downloaded or it must be
+  compiled (or both).  there are at present three download.sh scripts
+  (in the examples) which can be used for inspiration and guidance.
+
 * pyjamas UI development: follow GWT source code as closely as possible.
   make use of java2py.py (in contrib) to do 95% of the conversion work for
   you.  don't just follow the GWT API: do _literally_ "blindly" follow