[setup] Include README in MANIFEST

[jeffreythewang]

Since we've added README content to the setup description (in #5226), README.md must be in MANIFEST.in.

@xrmx @mistercrunch

[codecov-io]

Codecov Report

Merging #5531 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5531   +/-   ##
=======================================
  Coverage   63.31%   63.31%           
=======================================
  Files         349      349           
  Lines       22181    22181           
  Branches     2463     2463           
=======================================
  Hits        14044    14044           
  Misses       8123     8123           
  Partials       14       14

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