readme oversized gif fix

[fragmuffin]

Serving of images > 10MB seems to be denied by github.io.
Similar to issue: utopian-io/utopian.io#63

Changed link to jupyter showcase gif to a png with a play button overlay.

Personally: I think this is better than forcing our audience to download the 57MB gif

[AppVeyorBot]

✅ Build cadquery 1.0.85 completed (commit 09cbd9b97e by @fragmuffin)

[codecov-io]

Codecov Report

Merging #294 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #294   +/-   ##
=======================================
  Coverage   92.82%   92.82%           
=======================================
  Files          10       10           
  Lines        2215     2215           
=======================================
  Hits         2056     2056           
  Misses        159      159

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[coveralls]

Coverage remained the same at 92.28% when pulling f4f6417 on fragmuffin:feature/readme-rst into 938261b on dcowden:master.

[AppVeyorBot]

✅ Build cadquery 1.0.86 completed (commit 79702b6b0c by @fragmuffin)

[jmwright]

Thanks @fragmuffin