add contribution guidelines

CONTRIBUTING.md

@@ -0,0 +1,50 @@
+# How to contribute
+
+Please follow these simple guidelines,
+and question them if they don't make sense.
+
+
+## Overview
+
+* the `master` branch
+    * contains the source = [http.fsm.cosmogol](http.fsm.cosmogol) (and [http.graffle](http.graffle) for now)
+    * contains the distribution = the auto-generated [http.fsm.json](http.fsm.json)
+and [http.png](http.png)
+    * contains the documentation = the markdown files
+    * the source (WIP) and the distribution may obviously be out-of-sync in the
+      HEAD
+* the `generator` branch
+    * contains support scripts to regenerate the distribution in the `master`
+      branch based on the source files in the same `master` branch
+
+Corollary:
+
+* if you want to add/change the FSM, use `master` as base, and only submit changes to the source or documentation
+
+
+## How to prepare
+
+* You need a [GitHub account](https://github.com/signup/free)
+* [Submit an issue](https://github.com/for-GET/http-decision-diagram/issues) if
+  the not submitted beforehand.
+	* Describe the issue.
+	* Ensure that you're using the latest version.
+* Fork the repository on GitHub
+
+
+## Make Changes
+
+* In your forked repository, create a topic branch for your upcoming patch based
+  on one of the two main branches: `master` or `generator`.
+* Make commits of logical units and describe them properly.
+
+
+## Submit Changes
+
+* Push your changes to a topic branch in your fork of the repository.
+* Open a pull request to the original repository and choose the right original branch you want to patch.
+* If not done in commit messages (which you really should do) please reference and update your issue with the code changes.
+* Even if you have write access to the repository, do not directly push or merge pull-requests. Let another team member review your pull request and approve.
+
+
+Note: these guidelines are closely following [vagrant's contributing guidelines](https://raw.github.com/mitchellh/vagrant/master/CONTRIBUTING.md)