Contribution guide link

This is shown at the creation of new issues and new merge requests, when
the repository has a contribution guide file.

Fixes feedback item: http://feedback.gitlab.com/forums/176466-general/suggestions/5529802-show-a-link-to-the-contributing-file-when-creating

Screenshot:
![screenshot 2014-02-22 16 39 44](https://f.cloud.github.com/assets/1362793/2238027/999ddd60-9bd7-11e3-8275-2e7400264b7a.png)