Add instruction to install from the source

[watarungurunnn]

pip installs older version which doesn't work as in the instruction, so I added instruction to install from the source.

[codecov-commenter]

Codecov Report

Merging #73 (e1cdd48) into master (e3c4a1c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #73   +/-   ##
=======================================
  Coverage   86.22%   86.22%           
=======================================
  Files          12       12           
  Lines         421      421           
=======================================
  Hits          363      363           
  Misses         58       58           

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