contributing: remove C4 reference

Instead of referring to a large set of rules in an external document,
we now have sufficient guidelines in our own document to help
contributors. Our document emphasizes good code quality.

The rules and guidelines in our document are generic and have been
inspired by other large open source projects such as Mercurial, Git,
and OpenStack:

  http://mercurial.selenic.com/wiki/ContributingChanges
  https://github.com/git/git/blob/master/Documentation/SubmittingPatches
  https://wiki.openstack.org/wiki/GitCommitMessages

The idea is that this process should be familiar to other
contributors, while ensuring that we maintain high code and design
quality.
contributing: add link to contact page
contributing: new heading for detailed Git tips

The idea is to keep the guidelines at the top light-weight and easy to
read. The more heavy Git commands are then moved to the bottom part.
contributing: move "git rebase -i" text to bottom

Separating the detailed commands from the guideline above allows us to
expand on the commands without fearing that we obscure the message in
the guidelines.
contributing: move "git commit --amend" text to bottom

This also makes the guideline above more clear.