Add CONTRIBUTING.md

See https://github.com/blog/1184-contributing-guidelines
Hopefully this will miminise the amount of pull requests we keep
getting.

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Apache Cassandra and Pull Requests
+
+Apache Cassandra doesn't use GitHub pull requests as part of the development process.
+In fact, this repository is a GitHub mirror of [the official repo](https://git-wip-us.apache.org/repos/asf/cassandra.git). The development team has no control over it. We cannot merge or close any pull requests opened for the apache/cassandra repository, so please don't open them.
+
+# How to Contribute
+
+Use [Cassandra JIRA](https://issues.apache.org/jira/browse/CASSANDRA/) to create an issue, then either attach a patch or post a link to a GitHub branch with your changes.
+
+# Useful Links
+
+- How you can contribute to Apache Cassandra [presentation](http://www.slideshare.net/yukim/cassandrasummit2013) by Yuki Morishita
+- Code style [wiki page](https://wiki.apache.org/cassandra/CodeStyle)
+- Running Cassandra in IDEA [guide](https://wiki.apache.org/cassandra/RunningCassandraInIDEA)
+- Running Cassandra in Eclipse [guide](https://wiki.apache.org/cassandra/RunningCassandraInEclipse)
+- Cassandra Cluster Manager - [CCM](https://github.com/pcmanus/ccm) and a guide [blog post](http://www.datastax.com/dev/blog/ccm-a-development-tool-for-creating-local-cassandra-clusters)
+- Cassandra Distributed Tests aka [dtests](https://github.com/riptano/cassandra-dtest)