add contributing file

CONTRIBUTING.rst

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+You want help and contribute? Perfect!
+--------------------------------------
+
+*These are some contribution examples:*
+
+* Reporting issues to the bugtracker.
+
+* Submitting pull requests from a forked icalendar repo.
+
+* Extending the documentation.
+
+* Sponsor a Sprint (http://plone.org/events/sprints/whatis)
+
+
+*For pull requests, keep this in mind:*
+
+* Add a test which proves your fix and make it pass.
+
+* Describe your change in docs/changelog.rst 
+
+* Add yourself to the docs/credits.rst