Add deprecation notice to docs.

[tswast]

oauth2client is now deprecated, but when I look at https://oauth2client.readthedocs.io/en/latest/, I wouldn't know.

[techiesse]

Also in the api client documentation it is recomending the deprecated version: https://developers.google.com/api-client-library/python/guide/django

[parity3]

Is there a blog post or a rant from someone making the decision to deprecate an entire library silently without updating the documentation of several official google docs?

[theacodes]

@parity3 there's some google-internal documentation on why we deprecated. I'll get a distilled version published on the new library's documentation.

[theacodes]

See googleapis/google-auth-library-python#165

[theacodes]

Thank you for creating this issue, however, this project is deprecatedand we will only be addressing critical security issues. You can read moreabout this deprecation here.

If you need support or help using this library, we recommend that you ask yourquestion on StackOverflow.

If you still think this issue is relevant and should be addressed, pleasecomment and let us know!

[tswast]

Running a script @theacodes ? This does appear to still be relevant, as I still don't see a deprecation notice linking to google-auth at https://oauth2client.readthedocs.io/en/latest/

[theacodes]

It's in our readme, but not in docs.

Fixed it 0d1c814