Quick start:

1) git clone git@github.com:ChibiOS/ChibiOS-gitmain.git
2) cd ChibiOS-gitmain && \
   git submodule add git@github.com:ChibiOS/ChibiOS-Contrib.git community
Step #2 is optional if you interested in community code.

Contributing:

to ChibiOS-gitmain
1) fork git repository at your own account
2) make fix and test it
3) submit patch here http://sourceforge.net/projects/chibios/
   or here http://forum.chibios.org

to ChibiOS-Contrib
1) fork git repository at your own account
2) develop feature
3) send pull request as usual

Links:
http://chibios.org/dokuwiki/doku.php?id=chibios:guides:style_guide