Mention the schwab-py API and how tda-api will not be supported following TD Ameritrade's full transition to Schwab.

[HenryAsa]

Is your feature request related to a problem? Please describe.
With the upcoming Schwab transition from TD Ameritrade, I am proposing that we mention the new schwab-py repository in the README.rst file so people know that this API is slowly being phased-out and transitioned to the new schwab-py API.

Describe the solution you'd like
Add a statement toward the top of the README.rst file stating that this API is slowly being phased out and that the new schwab-py API should be referenced for the most up-to-date information/features.