Updated contribution information to include Gerrit and code style

As patches must be contributed by means of Gerrit as decribed in [1],
the file CONTRIBUTING.md needs some guidance on this. Moreover, some
information on code style has been added as well.

[1] https://www.eclipse.org/projects/handbook/#resources-source

Change-Id: Ib415a548f39e3c6b5025d89873d7a85b71ea5d5c
1 file changed