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
diff --git a/README.md b/README.md
index d1d7d3a..0fae27d 100644
--- a/README.md
+++ b/README.md
@@ -66,4 +66,6 @@
 
 ## 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.