Add CONTRIBUTING file.

If the CONTRIBUTING[.md] file exists, Github will show a link to it to
anyone who files an issue on Github or opens a pull request on
Github. This files points folks to the OpenStack wiki page which
provides the proper instructions on how to contribute to OpenStack.

See also <https://github.com/blog/1184-contributing-guidelines>.

Change-Id: I8fef06b71aa993c4c68a1ec92c39a540318aaf31

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+If you would like to contribute to the development of OpenStack,
+you must follow the steps in the "If you're a developer, start here"
+section of this page: [http://wiki.openstack.org/HowToContribute](http://wiki.openstack.org/HowToContribute#If_you.27re_a_developer.2C_start_here:)
+
+Once those steps have been completed, changes to OpenStack
+should be submitted for review via the Gerrit tool, following
+the workflow documented at [http://wiki.openstack.org/GerritWorkflow](http://wiki.openstack.org/GerritWorkflow).
+
+Pull requests submitted through GitHub will be ignored.
+
+Bugs should be filed [on Launchpad](https://bugs.launchpad.net/cinder),
+not in GitHub's issue tracker.