Add contributing file

CONTRIBUTING.rst

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+==========================
+How to contribute to Flask
+==========================
+
+Thanks for considering contributing to Flask. 
+
+Support questions
+=================
+
+Please, don't use the issue tracker for this. Check whether the ``#pocoo`` IRC
+channel on Freenode can help with your issue. If your problem is not strictly
+Werkzeug or Flask specific, ``#python`` is generally more active.
+`StackOverflow <https://stackoverflow.com/>`_ is also worth considering.
+
+Reporting issues
+================
+
+- Under which versions of Python does this happen? This is even more important
+  if your issue is encoding related.
+
+- Under which versions of Werkzeug does this happen? Check if this issue is
+  fixed in the repository.
+
+Submitting patches
+==================
+
+- Include tests if your patch is supposed to solve a bug, and explain
+  clearly under which circumstances the bug happens. Make sure the test fails
+  without your patch.
+
+- Try to follow `PEP8 <http://legacy.python.org/dev/peps/pep-0008/>`_, but you
+  may ignore the line-length-limit if following it would make the code uglier.
+
+
+Running the testsuite
+---------------------
+
+You probably want to set up a `virtualenv
+<http://virtualenv.readthedocs.org/en/latest/index.html>`_.
+
+The minimal requirement for running the testsuite is ``py.test``.  You can
+install it with::
+
+    pip install pytest
+
+Then you can run the testsuite with::
+
+    py.test
+
+With only py.test installed, a large part of the testsuite will get skipped
+though.  Whether this is relevant depends on which part of Werkzeug you're
+working on.  Travis is set up to run the full testsuite when you submit your
+pull request anyways.
+
+If you really want to test everything, you will have to install ``tox`` instead
+of ``pytest``. Currently we're depending on a development version of Tox
+because the released version is missing features we absolutely need. You can
+install it with::
+
+    pip install hg+https://bitbucket.org/hpk42/tox
+
+The ``tox`` command will then run all tests against multiple combinations
+Python versions and dependency versions.