Add contribution guide

CONTRIBUTING.md

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+We have three Branches Up and Running:
+
+MASTER:
+As soon as enough features are completed so that we have a fully implemented 
+version up and running which contains most/nearly all/all of the function, this 
+will be merged into master.
+Of course some stable releases will also be merged into master to get a rough 
+overview of where we are.
+
+FEATURE:
+All Features that are completed or Milestones that are reached will be merged
+into the feature branch. Version numbering should only change in the second
+digit (e.g. 0.1.0 --> 0.2.0).
+Software pushed to this branch should be a possible stable software release
+candidate. 
+NOTE: Possible Crashes should of course be fixed, which will result in the
+last digit being edited into the real release versioning.
+
+DEVEL: 
+All possible instable intermidiate states should be pushed to devel
+branch. Version Number is only allowed to be changed in the last digit
+(e.g. 0.0.1 --> 0.0.2). As soon as a feature is completed -> Merge to 
+FEATURE branch.
+
+More Branches might be added for some special features or to have a trial
+branch to try some new features or libraries.
+These branches MIGHT get deleted afterwards except the feature will be
+integrated into the final software, then the branch will be left open, to
+see the progrewss of the project.