docs: add contribution guide (#305)

*  Add partial conttribution guide
*  Document rough coding rules
*  Document commit rules
*  Document semantic-release

Fixes #303

Author: marionebl

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# Contribution guide
+
+## Developing commitizen
+
+You consider contributing changes to commitizen – thank you!
+Please consider these guidelines when filing a pull request:
+
+*  Commits follow the Angular commit convention
+*  JavaScript is written using ES2015 features
+*  4 spaces indentation
+*  Features and bug fixes should be covered by test cases
+
+## Creating releases
+
+commitizen uses [semantic-release](https://github.com/semantic-release/semantic-release)
+to release new versions automatically.
+
+*  Commits of type `fix` will trigger bugfix releases, think `0.0.1`
+*  Commits of type `feat` will trigger feature releases, think `0.1.0`
+*  Commits with `BREAKING CHANGE` in body or footer will trigger breakgin releases, think `1.0.0`
+
+All other commit types wil trigger no new release.