DOC: add a CONTRIBUTING file, which will be displayed when opening PRs.

[ci skip]

CONTRIBUTING.md

@@ -0,0 +1,66 @@
+==================================
+PyWavelets pull request guidelines
+==================================
+
+Pull requests are always welcome, and the PyWavelets community appreciates
+any help you give.
+
+When submitting a pull request, we ask you check the following:
+
+1. **Unit tests**, **documentation**, and **code style** are in order.
+   See below for details.
+
+   It's also OK to submit work in progress if you're unsure of what
+   this exactly means, in which case you'll likely be asked to make
+   some further changes.
+
+2. The contributed code will be **licensed under PyWavelets's license**,
+   (the MIT license, https://github.com/PyWavelets/pywt/blob/master/LICENSE).
+   If you did not write the code yourself, you ensure the existing
+   license is compatible and include the license information in the
+   contributed files, or obtain a permission from the original
+   author to relicense the contributed code.
+
+
+Coding guidelines
+=================
+
+1. Unit tests
+    In principle you should aim to create unit tests that exercise all the code
+    that you are adding.  This gives some degree of confidence that your code
+    runs correctly, also on Python versions and hardware or OSes that you don't
+    have available yourself.  An extensive description of how to write unit
+    tests is given in the NumPy `testing guidelines`_.
+
+2. Documentation
+    Clear and complete documentation is essential in order for users to be able
+    to find and understand the code.  Documentation for individual functions
+    and classes -- which includes at least a basic description, type and
+    meaning of all parameters and returns values, and usage examples in
+    `doctest`_ format -- is put in docstrings.  Those docstrings can be read
+    within the interpreter, and are compiled into a reference guide in html and
+    pdf format.  Higher-level documentation for key (areas of) functionality is
+    provided in tutorial format and/or in module docstrings.  A guide on how to
+    write documentation is given in `how to document`_.
+
+3. Code style
+    Uniformity of style in which code is written is important to others trying
+    to understand the code.  PyWavelets follows the standard Python guidelines
+    for code style, `PEP8`_.  In order to check that your code conforms to
+    PEP8, you can use the `pep8 package`_ style checker.  Most IDEs and text
+    editors have settings that can help you follow PEP8, for example by
+    translating tabs by four spaces.  Using `pyflakes`_ to check your code is
+    also a good idea.
+
+
+.. _PEP8: http://www.python.org/dev/peps/pep-0008/
+
+.. _pep8 package: http://pypi.python.org/pypi/pep8
+
+.. _how to document: https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt
+
+.. _doctest: http://www.doughellmann.com/PyMOTW/doctest/
+
+.. _pyflakes: http://pypi.python.org/pypi/pyflakes
+
+.. _testing guidelines: https://github.com/numpy/numpy/blob/master/doc/TESTS.rst.txt