Added a simple contributing document.

CONTRIBUTING.md

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+If you have a bugfix or new feature that you would like to contribute to
+elasticsearch-py, please find or open an issue about it first. Talk about what
+you would like to do. It may be that somebody is already working on it, or that
+there are particular issues that you should know about before implementing the
+change.
+
+We enjoy working with contributors to get their code accepted. There are many
+approaches to fixing a problem and it is important to find the best approach
+before writing too much code.
+
+The process for contributing to any of the Elasticsearch repositories is similar.
+
+1. Please make sure you have signed the [Contributor License
+   Agreement](http://www.elasticsearch.org/contributor-agreement/). We are not
+   asking you to assign copyright to us, but to give us the right to distribute
+   your code without restriction. We ask this of all contributors in order to
+   assure our users of the origin and continuing existence of the code. You only
+   need to sign the CLA once.
+
+2. Run the test suite to ensure your changes do not break existing code:
+
+    ````
+    git submodules init
+    python setup.py test
+    ````
+
+3. Rebase your changes.
+   Update your local repository with the most recent code from the main
+   elasticsearch-py repository, and rebase your branch on top of the latest master
+   branch. We prefer your changes to be squashed into a single commit.
+
+4. Submit a pull request. Push your local changes to your forked copy of the
+   repository and submit a pull request. In the pull request, describe what your
+   changes do and mention the number of the issue where discussion has taken
+   place, eg “Closes #123″.  Please consider adding or modifying tests related to
+   your changes.
+
+Then sit back and wait. There will probably be discussion about the pull
+request and, if any changes are needed, we would love to work with you to get
+your pull request merged into elasticsearch-py.
+