[MINOR] Update README.rst

[cakiki]

• The package is already pip installable.
• Simplify pip install from github

[cakiki]

Though maybe "manually" should be changed to "from source"?

[lmcinnes]

Yesm I think changing to "from source" makes sense here.

[codecov-commenter]

Codecov Report

Merging #117 (e6ef69c) into master (6afac93) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #117   +/-   ##
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  Coverage   91.15%   91.15%           
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  Files          34       34           
  Lines        4849     4849           
=======================================
  Hits         4420     4420           
  Misses        429      429           

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[lmcinnes]

Thanks!