Add a CONTRIBUTING.md

As was recently discussed, we would like to adopt a Gitlab Flow based branching model for Amavis, because it allows supporting multiple minor releases with continuous bugfix releases, while the Git Flow branching model originally proposed in #42 (closed) only allows supporting one release at a time. Gitlab Flow is also simpler to use than Git Flow for contributors who are unfamiliar with the used branching model, as it always requires changes to be merged into master, which is what most people are used to anyway, whereas Git Flow requires people to merge into different branches depending on whether they are making a feature or a hotfix.

On the flip side Gitlab Flow comes with a slightly more complicated release process than Git Flow. But all people currently expected to manage releases agreed that this is a sensible way forward.

The documentation this MR adds explains the procedure for opening a merge request when making a contribution.

If this MR doesn't face a lot of opposition and gets approved/merged, I will go ahead and set up the branches correctly to tackle #43 (closed). As I currently see it, this only requires deleting the development branch and changing existing merge requests to target master instead.