Update readme

[usulpro]

Issues:

• Complete the deprecation of "built-in addons" #1268 (partially)

• Versioned Documentation #1357 (comment) We don't have any link in the readme.md to clarify that migration.md exist

• We don't have any "live demo" links on the first page. We had, but it was shifted to a deeper level of docs then.

What I did

• add a link to a links addon :)

• add a note about v2 -> v3 mirgation

• add a link to storybook "live" examples

How to test

1. Read

2. Check links

[codecov]

Codecov Report

Merging #1380 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1380   +/-   ##
=======================================
  Coverage   14.35%   14.35%           
=======================================
  Files         201      201           
  Lines        4613     4613           
  Branches      506      494   -12     
=======================================
  Hits          662      662           
- Misses       3508     3520   +12     
+ Partials      443      431   -12

Impacted Files	Coverage Δ
addons/storyshots/src/storybook-channel-mock.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_keyevents.js	33.33% <0%> (ø)	⬆️
addons/knobs/src/components/PropForm.js	8.51% <0%> (ø)	⬆️
addons/knobs/src/KnobManager.js	38.57% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_routing.js	28.94% <0%> (ø)	⬆️
lib/ui/src/modules/ui/libs/filters.js	41.66% <0%> (ø)	⬆️
...codemod/src/transforms/update-organisation-name.js	40.62% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/down_panel.js	23.52% <0%> (ø)	⬆️
... and 21 more

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