Bring back some of the contribution guidelines

We had a couple of create issue->immediately send PR submissions
again. Let's explain a bit more what we expect from people.

CONTRIBUTING.md

@@ -1,4 +1,5 @@
 # Contributing to OSEM
+
 We here at OSEM are open for all types of contributions from anyone. Tell us about our [issues/ideas](https://github.com/openSUSE/osem/issues/new), propose code changes via [pull requests](https://help.github.com/articles/using-pull-requests) or contribute artwork and documentation.
 
 We welcome all new developers and are also prepared to mentor you through your first contributions! All OSEM maintainers are seasoned developers and have participated in mentoring programs, such as [GSoC](https://summerofcode.withgoogle.com/) and [RGSoC](https://railsgirlssummerofcode.org/).
@@ -13,6 +14,7 @@ We need your input and contributions to OSEM. In particular we seek the followin
 Read this guide on how to do that.
 
 ## How to contribute code
+
 1. Fork the repository and make a pull-request with your changes
     1. Make sure that the test suite passes and that you comply to our code style
     1. Please increase code coverage with your pull request
@@ -20,10 +22,59 @@ Read this guide on how to do that.
     1. If you are already a contributor and you get a positive review, you can merge your pull-request yourself
     1. If you are not already a contributor, one of the existing contributors will merge your pull-request
 
+**However, please bear in mind the following things:**
+
+### Discuss Large Changes in Advance
+
+If you see a glaring flaw within OSEM, resist the urge to jump into the
+code and make sweeping changes right away. We know it can be tempting, but
+especially for large, structural changes it's a wiser choice to first discuss
+them in the [issue list](https://github.com/openSUSE/osem/issues).
+
+A good rule of thumb, of what a *structural change* is, is to estimate how much
+time would be wasted if the pull request was rejected. If it's a couple of minutes
+then you can probably dive head first and eat the loss in the worst case. Otherwise,
+making a quick check with the other developers could save you lots of time down the line.
+
+Why? It may turn out that someone is already working on this or that someone already
+has tried to solve this and hit a roadblock, maybe there even is a good reason
+why this particular flaw exists? If nothing else, a discussion of the change will
+usually familiarize the reviewer with your proposed changes and streamline the
+review process when you finally create a pull request.
+
+### Small Commits & Pull Request Scope
+
+A commit should contain a single logical change, the scope should be as small
+as possible. And a pull request should only consist of the commits that you
+need for your change. If it's possible for you to split larger changes into
+smaller blocks please do so.
+
+Why? Limiting the scope of commits/pull requests makes reviewing much easier.
+Because it will usually mean each commit can be evaluated independently and a
+smaller amount of commits per pull request usually also means a smaller amount
+of code to be reviewed.
+
+### Proper Commit Messages
+
+We are keen on proper commit messages because they will help us to maintain
+this code in the future. We define proper commit messages like this:
+
+* The title of your commit message summarizes **what** has been done
+* The body of your commit message explains **why** you have done this
+
+If the title is to small to explain **what** you have done, then you can of course
+elaborate about it in the body. Please avoid explaining *how* you have done this,
+we are developers too and we see the diff, if we do not understand something we will
+ask you in the review.
+
+Additional to **what** and **why** you should explain potential **side-effects** of
+this change, if you are aware of any.
+
+The content is most important, but please also use a [proper style](https://github.com/openSUSE/osem/wiki/Commit-message-guidelines).
+
 ## Development Environment
 
-### with docker-compose
+To isolate your host system from OSEM development we have prepared a container
-To isolate you host system from OSEM development we have prepared a container
 based development environment, based on [docker](https://www.docker.com/) and
 [docker-compose](https://docs.docker.com/compose/). Here's a step by step guide
 how to set it up.
@@ -66,23 +117,25 @@ sed "s/13042/`id -u`/" docker-compose.override.yml.example > docker-compose.over
     docker-compose exec osem /bin/bash -l
     ```
 
+1. Want to know more?
+    In our wiki you can find more information about what is possible in our development environment,
+    how we work with each other on github or other topics of interest for OSEM developers.
+
+    https://github.com/openSUSE/osem/wiki
+
 ## How to contribute translations
+
 Please refer to our [translation guide](https://github.com/openSUSE/osem/wiki/Translation) in the wiki.
 
-## Want to know more?
-In our wiki you can find more information about what is possible in our development environment,
-how we work with each other on github or other topics of interest for developers. Visit us at
->>>>>>> Introduces docker-compose to the contribution guide
-
-https://github.com/openSUSE/osem/wiki
-
 ## Code of Conduct
+
 OSEM is part of the openSUSE project. We follow all the
 [openSUSE Guiding Principles!](http://en.opensuse.org/openSUSE:Guiding_principles)
 If you think someone doesn't do that, please let us know at maintainers@osem.io or
 address your concerns to the [openSUSE Board](https://en.opensuse.org/openSUSE:Board).
 
 ## Contact
+
 GitHub issues and pull requests are the primary way for communicating about specific proposed
 changes to this project. If you have other questions feel free to subscribe to
 the [opensuse-web@opensuse.org](http://lists.opensuse.org/opensuse-web/)