add step for import addon-action to addons.js file before start using it

[natsathorn]

Issue: When I try using Storybook I was following the step in this repository but it has no step that told me to import addon-action to addons.js file. So, I've spend a lot of time to find out why I have no action logger

What I did

Add the step that told user to import @storybook/addon-actions/register to addons.js before start using action on components

How to test

Nothing to test, MD file will break nothing.

[codecov]

Codecov Report

Merging #1327 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1327   +/-   ##
=======================================
  Coverage   13.76%   13.76%           
=======================================
  Files         201      201           
  Lines        4620     4620           
  Branches      506      503    -3     
=======================================
  Hits          636      636           
- Misses       3543     3557   +14     
+ Partials      441      427   -14

Impacted Files	Coverage Δ
lib/components/src/demo/Welcome.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/WrapStory.js	12% <0%> (ø)	⬆️
addons/info/src/components/Props.js	0% <0%> (ø)	⬆️
addons/info/src/components/PropVal.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/client_api.js	39.28% <0%> (ø)	⬆️
addons/knobs/src/KnobManager.js	38.57% <0%> (ø)	⬆️
app/react/src/client/preview/error_display.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_routing.js	28.94% <0%> (ø)	⬆️
.../ui/src/modules/ui/components/layout/dimensions.js	15.62% <0%> (ø)	⬆️
app/react/src/server/build.js	0% <0%> (ø)	⬆️
... and 17 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b975fc3...62b2651. Read the comment docs.

[shilman]

LGTM! thanks!