added CONTRIBUTING guidelines

CONTRIBUTING.rst

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+Contributing to Astroquery
+==========================
+The first official release of astroquery is scheduled for late summer
+(August/September) 2013.  There are therefore two sets of contribution
+guidelines below.
+
+Please see `astropy's contributing guildelines
+<http://www.astropy.org/contributing.html>`__ for a general guide to the
+workflow involving git, etc.  Everything below is astroquery-specific.
+
+Prior to first release
+----------------------
+We welcome any and all new features!  If you have your own little query tool
+you wrote to access some obscure service, feel free to clean it up a little and
+submit it as a pull request (PR)!  At this stage, we are mostly looking to be
+as inclusive as possible.
+
+Tests are welcome and encouraged, but can be built up over time.  At least one
+example use is necessary, however!
+
+The minimum requirements for a new feature are:
+
+ * Add the feature as a subdirectory of astroquery with at least an
+   ``__init__.py`` and a ``core.py``::
+ 
+     astroquery/feature
+     astroquery/feature/__init__.py
+     astroquery/feature/core.py
+
+ * Add a ``tests/`` directory with at least one test::
+ 
+     astroquery/feature/tests
+     astroquery/feature/tests/__init__.py
+     astroquery/feature/tests/test_feature.py
+
+ * Add some documentation - at least one example, but it can be sparse at first::
+ 
+     docs/astroquery/feature.rst
+
+Ideally, we'd like the test to pass on all branches on travis-ci, but until
+that's true of master (**keep this updated!! as of 5/28/2013, tests do not
+pass**), it cannot be expected of PRs either.
+
+
+After first release
+-------------------
+Once the first release has occurred, it will be necessary to follow the strict
+API guidelines laid out in :doc:`api`.
+
+Still feel free to submit PRs that do not fully follow the guidelines - at
+least it will make us aware of the need for a new feature.  However, please be
+willing to work with us to make the new feature conform to astroquery
+standards: no PRs will be accepted that do not conform.