Update README

[keynmol]

A drive-by PR as I browse things published for SN :)

• Supported Scala versions
• Supported platforms
• Versions in installation instructions

[johnynek]

Thanks so much!

[codecov-commenter]

Codecov Report

Merging #387 (fe34eb0) into master (4e0b463) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #387   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files           8        8           
  Lines         412      412           
  Branches       41       41           
=======================================
  Hits          410      410           
  Misses          2        2           

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