Add MAINTAINERS and CONTRIBUTING guidelines.

CONTRIBUTING

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+Contributing guidelines
+=======================
+
+Wanna contribute to the Postgres Operator? Yay - here is how!
+
+Reporting issues
+----------------
+
+If you have a question about patroni or have a problem using it, please read the :ref:`README <readme>` before filing an issue.
+Also double check with the current issues on our `Issues Tracker <https://github.com/zalando-incubator/postgres-operator/issues>`__.
+
+Contributing a pull request
+---------------------------
+
+1) Submit a comment to the relevant issue or create a new issue describing your proposed change.
+2) Do a fork, develop and test your code changes.
+3) Include documentation
+4) Submit a pull request.
+
+You'll get feedback about your pull request as soon as possible.
+
+Happy Operator hacking ;-)

MAINTAINERS

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+Murat Kabilov  <murat.kabilov@zalando.de>
+Oleksii Kliukin <oleksii.kliukin@zalando.de>