Add header to readme with link to documentation portal

[JakeSCahill]

Added a consistent header that will be used across all our client libraries. This header has a clear link to the official docs.

[lzpap]

Keep README.rst as it is needed for RTD build. Move it under docs.

[lzpap]

README.rst is needed to build the documentation for ReadTheDocs.
We could move this file under docs folder, and modify docs/index.rst to contain

.. include:: README.rst

[lzpap]

One more thing to make the build pass: add docs/ to the filename in setup.py like this:

# Load long description for PyPI.
with open('docs/README.rst', 'r', 'utf-8') as f:  # type: StreamReader
    long_description = f.read()