Document revocation flow

[justin-barca-at-camis]

Expected Behavior

[REQUIRED] Describe expected behavior

There should be clear instructions on how to revoke a token.

Describe the problem

There's no mention of token revocation in the readme and the electron demo just disposes of the tokens in memory but does not revoke the token

[REQUIRED] Actual Behavior

Sign out in electron demo should revoke tokens

[REQUIRED] Steps to reproduce the behavior

N/A

[REQUIRED] Environment

• AppAuth-JS version: current
• AppAuth-JS Environment (Node, Browser (UserAgent), ...): N/A
• Source code snippts (inline or JSBin)

NOTE: there is mention of revocation in #71