Document how to run tests locally in README.rst

[mattcarrollcode]

Running tests for this library is currently not documented in README.rst despite some mentions of how to run tests in CONTRIBUTING.rst there is no documentation for how to setup your locally environment to fully test the library.

This is a request for documentation on how to run all tests for this library locally.

[mattcarrollcode]

Closing this to update this documentation to be in the template for CONTRIBUTING.rst instead of README.rst: https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/python_library/CONTRIBUTING.rst