added a CONTRIBUTING doc

CONTRIBUTING.md

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+
+Contributing code
+=================
+
+How to contribute
+-----------------
+
+The preferred way to contribute to scikit-learn is to fork the 
+[main repository](http://github.com/bmcfee/librosa/) on
+GitHub:
+
+1. Fork the [project repository](http://github.com/bmcfee/librosa):
+   click on the 'Fork' button near the top of the page. This creates
+   a copy of the code under your account on the GitHub server.
+
+2. Clone this copy to your local disk:
+
+          $ git clone git@github.com:YourLogin/librosa.git
+          $ cd scikit-learn
+
+3. Create a branch to hold your changes:
+
+          $ git checkout -b my-feature
+
+   and start making changes. Never work in the ``master`` branch!
+
+4. Work on this copy on your computer using Git to do the version
+   control. When you're done editing, do:
+
+          $ git add modified_files
+          $ git commit
+
+   to record your changes in Git, then push them to GitHub with:
+
+          $ git push -u origin my-feature
+
+Finally, go to the web page of the your fork of the librosa repo,
+and click 'Pull request' to send your changes to the maintainers for
+review. This will send an email to the committers.
+
+(If any of the above seems like magic to you, then look up the 
+[Git documentation](http://git-scm.com/documentation) on the web.)
+
+It is recommended to check that your contribution complies with the
+following rules before submitting a pull request:
+
+-  All public methods should have informative docstrings with sample
+   usage presented.
+
+You can also check for common programming errors with the following
+tools:
+
+-  Code with good unittest coverage (at least 80%), check with:
+
+          $ pip install nose coverage
+          $ nosetests --with-coverage path/to/tests_for_package
+
+-  No pyflakes warnings, check with:
+
+           $ pip install pyflakes
+           $ pyflakes path/to/module.py
+
+-  No PEP8 warnings, check with:
+
+           $ pip install pep8
+           $ pep8 path/to/module.py
+
+-  AutoPEP8 can help you fix some of the easy redundant errors:
+
+           $ pip install autopep8
+           $ autopep8 path/to/pep8.py
+
+
+Documentation
+-------------
+
+You can edit the documentation using any text editor and then generate
+the HTML output by typing ``make html`` from the docs/ directory.
+The resulting HTML files will be placed in _build/html/ and are viewable 
+in a web browser. See the README file in the doc/ directory for more information.
+
+For building the documentation, you will need
+[sphinx](http://sphinx.pocoo.org/),
+[matplotlib](http://matplotlib.sourceforge.net/), and [scikit-learn](http://scikit-learn.org/).
+
+
+Note
+----
+This document was gleefully borrowed from [scikit-learn](http://scikit-learn.org/).