Add CONTRIBUTE.md to help new contributors. (#2581)

This file will be shown when people make new pull requests.

CONTRIBUTE.md

@@ -0,0 +1,25 @@
+Contributing to QGIS
+====================
+
+QGIS is an open source project and we appreciate contributions very much.
+
+Proper formatting
+-----------------
+
+Before making a pull request, please make sure your code is properly formatted
+by running the prepare commit script **before** issuing `git commit`.
+
+    ./scripts/prepare-commit.sh
+
+This can be automated by setting up the pre-commit hook properly.
+
+    ln -s scripts/prepare-commit.sh .git/hooks/pre-commit
+
+Getting your pull request merged
+--------------------------------
+
+This is a volunteer project, so sometimes it may take a little while to merge
+your pull request.
+
+There is a [guide with hints for getting your pull requests merged](https://github.com/m-kuhn/QGIS-Website/blob/qtcreator.rst/source/site/getinvolved/development/git.rst#pull-requests)
+in the developers guide.