snapshot testing inverse regex example documentation

[epicallan]

Issue:

What I did

Added documentation about using inverse regex to exclude stories from snapshot testing following this issue #892 (comment)

How to test

[codecov]

Codecov Report

Merging #1317 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1317   +/-   ##
=======================================
  Coverage   13.76%   13.76%           
=======================================
  Files         201      201           
  Lines        4620     4620           
  Branches      506      508    +2     
=======================================
  Hits          636      636           
- Misses       3538     3541    +3     
+ Partials      446      443    -3

Impacted Files	Coverage Δ
addons/knobs/src/components/PropForm.js	8.51% <0%> (ø)	⬆️
.../src/manager/containers/CommentsPanel/dataStore.js	34.78% <0%> (ø)	⬆️
addons/info/src/components/Props.js	0% <0%> (ø)	⬆️
addons/storyshots/src/require_context.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Select.js	7.93% <0%> (ø)	⬆️
app/react/src/client/preview/error_display.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/left_panel.js	20% <0%> (ø)	⬆️
addons/knobs/src/KnobStore.js	6.81% <0%> (ø)	⬆️
app/react/src/client/preview/init.js	0% <0%> (ø)	⬆️
...codemod/src/transforms/update-organisation-name.js	40.62% <0%> (ø)	⬆️
... and 18 more

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[tmeasday]

LGTM, thanks!

[epicallan]

@tmeasday LGTM indeed and you are welcome. Thanks for all your work as well.