Add CONTRIBUTING.md.

CONTRIBUTING.md

+# Contributing to Hypha
+
+First off, thanks for taking the time to contribute!
+
+Before you start working however, please read and follow this short guide.
+
+
+## Reporting Issues
+
+All changes in Hypha starts as an issue.
+
+- Pick the correct option for issue among the options your are given (bug report, feature request etc.).
+- Provide as much information as possible. There are templates for each issue type that will guide you.
+
+
+## Creating Pull Requests
+
+Found a bug and know how to fix it or built a new feature you want to add? Great! Please read on.
+
+- Each pull request need to be linked to an issue. If non exist, create one first.
+- Rebase your topic branch on top of the master branch before creating the pull
+ request.
+- One feature or fix per pull request.
+
+You find more information in the wiki page [Submitting changes](https://github.com/OpenTechFund/hypha/wiki/Submitting-changes)