Tweak README.md

[tkf]

• Link BangBang.jl as it uses setproperties
• Add a GitHub badge so that it's easy to jump to GitHub front page from the documentation.

[codecov-io]

Codecov Report

Merging #20 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #20   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          23     23           
=====================================
  Hits           23     23

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (6529367) to head (6634aee).
Report is 167 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff            @@
##            master       #20   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           23        23           
=========================================
  Hits            23        23           

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