Important note about refresh_token

[franciscocpg]

• npm test succeeds
• Pull request has been squashed into 1 commit
• I did NOT manually make changes to anything in apis/
• Code coverage does not decrease (if any source code was changed)
• Appropriate JSDoc comments were updated in source code (if applicable)
• Approprate changes to readme/docs are included in PR

I almost lost my mind trying to understand why refresh_tokens weren't in my response until I find #750 (comment).
This PR tries to make this more obvious at README.

[codecov-io]

Codecov Report

Merging #788 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #788   +/-   ##
=======================================
  Coverage   79.49%   79.49%           
=======================================
  Files           5        5           
  Lines         239      239           
  Branches       53       53           
=======================================
  Hits          190      190           
  Misses         30       30           
  Partials       19       19

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