Test for CONTRIBUTING.txt UI/UX in GitHub

Since this sandbox repository is mirrored to GitHub,
it is one of places to test for CONTRIBUTING.txt UI/UX.

More details:
http://lists.openstack.org/pipermail/openstack-dev/2016-December/109294.html

Change-Id: If61d2b8ed20752de64d0ae06b57152d7dec9f96a

CONTRIBUTING.txt

@@ -0,0 +1,18 @@
+If you would like to contribute to the development of OpenStack, you must
+follow the steps in this page:
+
+   http://docs.openstack.org/infra/manual/developers.html
+
+If you already have a good understanding of how the system works and your
+OpenStack accounts are set up, you can skip to the development workflow
+section of this documentation to learn how changes to OpenStack should be
+submitted for review via the Gerrit tool:
+
+   http://docs.openstack.org/infra/manual/developers.html#development-workflow
+
+Pull requests submitted through GitHub will be ignored.
+
+Bugs should be filed on Launchpad, not GitHub:
+
+   https://bugs.launchpad.net/oslo
+