Add a reference to nbclick to README

[dokempf]

This fixes #21

[codecov]

Codecov Report

Merging #22 (d74d9c9) into master (572dad0) will not change coverage.
The diff coverage is n/a.

❗ Current head d74d9c9 differs from pull request most recent head 442f0a8. Consider uploading reports for the commit 442f0a8 to get more accurate results

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   92.08%   92.08%           
=======================================
  Files           3        3           
  Lines         139      139           
=======================================
  Hits          128      128           
  Misses         11       11           

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[takluyver]

Thanks!