Quick start:

Assume you already have a github account. Lets your account name is "username".

First you need to fork 2 repos:

Make local clone of your fork

# git clone git@github.com:username/ChibiOS.git

Make local branch for your feature

# cd ChibiOS
# git branch feature
# git checkout feature

There is no code from ChibiOS-contrib in your repo. We have to connect ChibiOS-contrib as a git submodule

# git submodule add git@github.com:username/ChibiOS-Contrib.git community

Make branch again, but now in freshly connected submodule

# cd community
# git branch feature
# git checkout feature

Hack, hack, hack... Commit our changes and push to server. Note: you have to do this for both repositories. First ChibiOS-Contrib

# cd community
# git add .
# git commit -m "Feature implemented"
# git push origin feature:feature

Now for parent repo

# cd ..
# git add .
# git commit -m "Feature implemented"
# git push origin feature:feature

Now all your changes published on github. Just use web interface to send 2 pull requests: for ChibiOS-contrib and for ChibiOS.

Links: