docs: consolidating and updating the Contribution Guide (#964)

* Removing `.github/CONTRIBUTING.md`
* Creating new `CONTRIBUTING.rst` at the root of the repo that:
    * Links to the [Contribution Guide](http://googleapis.github.io/google-api-python-client/contributing.html) for basic housekeeping items such as CLA, Code of Conduct, Pull Requests, Style, Code Reviews.
    * Provides setup instructions and introduces Nox
* Modifying `README.md` to reference new `CONTRIBUTING.rst` 

Fixes: #963

Author: Dmitry Frenkel

.github/CONTRIBUTING.md

@@ -0,0 +1,61 @@
+How to Contribute
+=================
+
+We'd love to accept your patches and contributions to this project.
+There are a few guidelines described in our
+`Contribution Guide <http://google.github.io/google-api-python-client/contributing.html>`__
+that you need to follow.
+
+To summarize here: when contributing, please:
+
+* Sign Contributor License Agreement
+* Before making changes, file an issue
+* Fork this repository and use github pull requests for all submissions
+* Follow
+  `Contributor Code of Conduct
+  <https://github.com/googleapis/google-api-python-client/blob/master/CODE_OF_CONDUCT.md>`__
+  and `Community Guidelines <https://opensource.google/conduct/>`__
+* Follow `Google Python Style Guide <https://google.github.io/styleguide/pyguide.html>`__
+  and `this commit authoring style <http://chris.beams.io/posts/git-commit/#seven-rules>`__
+* Don't forget to write tests and update documentation!
+
+Setup Notes
+-----------
+
+Please follow these steps after forking and cloning the repository
+to make sure you can modify code and run tests with confidence::
+
+    # From the root dir of the cloned repository:
+    # Create Virtual Environment called env (you may choose your own name)
+    python3 -m venv env
+
+    # Activate virtual environment
+    source env/bin/activate
+
+    # Install this library as editable install
+    # (see https://pip.pypa.io/en/stable/reference/pip_install/#cmdoption-e)
+    python3 -m pip install -e .
+
+    # Install nox
+    python3 -m pip install nox
+
+We use `nox <https://nox.thea.codes/>`__ to instrument our tests.
+To test your changes, run unit tests with ``nox``::
+
+    # Run tests for all supported versions of Python and oauth2client:
+    nox
+    # Run tests for Python 3.7:
+    nox -s unit-3.7
+    # Run lint
+    nox -s lint
+
+
+.. note::
+
+  The unit tests and system tests are described in the
+  ``noxfile.py`` file in this directory. Nox will automatically
+  handle constriction of new virtual environments and installation
+  of the required test dependencies.
+
+For more information about Nox, including command-line usage, consult
+`nox documentation <https://nox.thea.codes/>`__.

CONTRIBUTING.rst

@@ -66,4 +66,6 @@ For development you will also need the following libraries:
 
 ## Contributing
 
-Please see the [contributing page](http://google.github.io/google-api-python-client/contributing.html) for more information. In particular, we love pull requests - but please make sure to sign the contributor license agreement.
+Please see our [Contribution Guide](CONTRIBUTING.rst). 
+In particular, we love pull requests - but please make sure to sign 
+the contributor license agreement.