Update README: mention that addon-links needs to be registered manually

[mrtnpro]

Issue:

What I did

Update README.md by mentioning that addon-links needs to be manually registered. I just ran into this and since the README didn't mention anything it cost me some time to figure out why I couldn't make it work.

How to test

N.A.

Is this testable with jest or storyshots?
N.A.
Does this need a new example in the kitchen sink apps?
N.A.
Does this need an update to the documentation?
N.A.
If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #1835 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1835   +/-   ##
=======================================
  Coverage   21.32%   21.32%           
=======================================
  Files         257      257           
  Lines        5739     5739           
  Branches      686      678    -8     
=======================================
  Hits         1224     1224           
+ Misses       4020     3996   -24     
- Partials      495      519   +24

Impacted Files	Coverage Δ
lib/ui/src/modules/ui/containers/layout.js	12.5% <0%> (ø)	⬆️
addons/knobs/src/react/WrapStory.js	12% <0%> (ø)	⬆️
addons/knobs/src/components/PropField.js	10.86% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	30% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_routing.js	28.04% <0%> (ø)	⬆️
lib/ui/src/modules/ui/routes.js	0% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/error_display.js	0% <0%> (ø)	⬆️
... and 19 more

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[usulpro]

Thanks @crtvhd!
Sorry that confused you with this outdated description!

You can use this addon without installing it.

Would you like to fix it as well, since it should be installed now?

[ndelangen]

I changed it @usulpro ✅

Merge when ready

[usulpro]

Why not as a devDependency? 😕

[ndelangen]

yeah that would be even better

[mrtnpro]

Would you like to fix it as well, since it should be installed now?

Well @ndelangen was faster :) Thanks!