build instructions for macOS

[fkr]

No description provided.

[codecov]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #9   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          16     16           
  Lines         302    302           
=====================================
  Hits          302    302

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[lambdafu]

There is probably a way to do this without environment variables, but any instructions are better than none! Thanks a lot!