Add viewport addon to the Addon Gallery

[mshaaban088]

Issue: viewport addon is not shown as part of the Addon Gallery

What I did

• I added it to the list with brief description (same as README file)

How to test

• Just review the change for grammar mistakes or typos

Is this testable with jest or storyshots?

• No

Does this need a new example in the kitchen sink apps?

• No

Does this need an update to the documentation?

• It is an update to the documentation, so no

If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #3106 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3106   +/-   ##
=======================================
  Coverage   36.08%   36.08%           
=======================================
  Files         438      438           
  Lines        9447     9447           
  Branches      881      893   +12     
=======================================
  Hits         3409     3409           
+ Misses       5497     5461   -36     
- Partials      541      577   +36

Impacted Files	Coverage Δ
app/polymer/src/server/utils.js	0% <0%> (-100%)	⬇️
...tories_panel/stories_tree/tree_decorators_utils.js	45.23% <0%> (ø)	⬆️
lib/components/src/table/cell.js	65.21% <0%> (ø)	⬆️
.../ui/src/modules/ui/components/layout/dimensions.js	15.62% <0%> (ø)	⬆️
lib/core/src/client/manager/preview.js	0% <0%> (ø)	⬆️
addons/info/src/components/types/OneOf.js	61.11% <0%> (ø)	⬆️
addons/info/src/components/types/PrettyPropType.js	86% <0%> (ø)	⬆️
addons/viewport/src/components/RotateViewport.js	22.72% <0%> (ø)	⬆️
addons/storyshots/src/rn/loader.js	53.33% <0%> (ø)	⬆️
addons/info/src/components/types/Enum.js	0% <0%> (ø)	⬆️
... and 60 more

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[mshaaban088]

I updated the snapshots for both react_native and react_native_scripts, as they both were depending on prop-types ^15.6.0, and a new patch version of prop-types was released which broke the tests

Nothing really has changed in prop-types apart from, let me quote it, license header (see #164)