Add install instruction

[pllim]

I wasn't sure if I am supposed to clone it or pip install but then I found it on PyPI.

[codecov]

Codecov Report

Merging #1 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #1   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files           4        4           
  Lines         132      132           
=======================================
  Hits          126      126           
  Misses          6        6

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