Add installation instructions

[hamelsmu]

Added installation instructions for theme

cc: @choldgraf @mmcky

closes executablebooks/meta#264

[codecov-io]

Codecov Report

Merging #88 (2fea2d3) into master (8d072ad) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   73.12%   73.12%           
=======================================
  Files           2        2           
  Lines         227      227           
=======================================
  Hits          166      166           
  Misses         61       61           

Flag	Coverage Δ
pytests	73.12% <ø> (ø)

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[mmcky]

thanks @hamelsmu sorry this took so long to merge.