Contribution Guide(s) need to be consolidated and updated

[DFrenkel]

This repo contains a brief Contribution Guide in the .github directory.

However, README references another Contribution Guide instead with similar, expanded, content.

Neither guides contain instructions for setting up this project to be able to do effective testing, such as:

---

Setup Instructions

Please follow these steps after 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 edible install
python3 -m pip install -e .

# Install nox
python3 -m pip install nox

From that point, you can just use nox to run linter and unit tests for all supported versions of Python and oauth2client defined in the noxfile.py
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 documentation.

---

I propose the following restructuring:

• Remove .github/CONTRIBUTING.md
• Create new CONTRIBUTING.rst at the root of the repo that:
  • Links to the Contribution Guide for basic housekeeping items such as CLA, Code of Conduct, Pull Requests, Style, Code Reviews.
  • Provides setup instructions above
• Modify README.md to reference new CONTRIBUTING.rst perhaps reinforcing again basic requirements as bullet points