Add demo gif to README

[chfw]

Before raising the PR, here is a check list:

• have you written unit tests for your code changes?
• have you updated the change log?
• can someone else understand your changes without your explanation?
• are you proud of your code changes?
• please add your name and github link to contributors.rst in alphabetical order.

[codecov-io]

Codecov Report

Merging #355 into dev will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             dev     #355      +/-   ##
=========================================
+ Coverage   96.3%   96.32%   +0.02%     
=========================================
  Files         55       55              
  Lines       2812     2832      +20     
=========================================
+ Hits        2708     2728      +20     
  Misses       104      104

Impacted Files	Coverage Δ
moban/constants.py	100% <0%> (ø)	⬆️

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