Add "Projects using d3-graphviz" section to README

[magjac]

And add graphviz-anywidget to that section, as suggested in https://github.com/magjac/d3-graphviz/issues/329.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (355158d) to head (97c07dd).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #331   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           39        39           
  Lines         2309      2309           
=========================================
  Hits          2309      2309           

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