Add a warning in 2.0.0.md about Promise.finally

[SebC99]

Whereas Parse.Promise.always, Promise.finally callback don't receive any arguments, and don't resolve with a new argument to pass to the next promise in chain.

[codecov]

Codecov Report

Merging #636 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #636   +/-   ##
=======================================
  Coverage   85.07%   85.07%           
=======================================
  Files          48       48           
  Lines        3853     3853           
  Branches      871      871           
=======================================
  Hits         3278     3278           
  Misses        575      575

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[flovilmart]

Thanks! That helps!