Add CONTRIBUTING.md

CONTRIBUTING.md

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+CONTRIBUTING
+============
+
+If you have any kind of feedback, simply create an issue on Github. This makes
+it easier for everyone to track and comment recent developments.
+
+If you want to contribute, please keep a few simple rules in mind:
+
+#### The commit
+
+- One commit per fix/addition.
+- The short commit message shouldn't be longer than 50 characters.
+- The short commit message must start with an uppercase character.
+- The short commit message must start with the affected section followed by a
+  colon (if it affects any section).
+- The short commit message must use the
+  [imperative](https://en.wikipedia.org/wiki/Imperative_mood).
+- The short commit message mustn't end with a `.`.
+- It's always a good idea to have a look at other commits to get an idea how to
+  format one's own commits messages.
+
+#### Example
+
+```
+Bad:  "added my new plugin"
+Good: "Plugins: add vim-newplugin"
+```