fix Storyshots README, add missing export for image snapshots

[thomasbertet]

Issue:

What I did

• Add missing export to README (& minor fix to other parts of README)

How to test

• Open the README and read it 😄
  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?
  Yep
  If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #2759 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2759   +/-   ##
=======================================
  Coverage   35.73%   35.73%           
=======================================
  Files         433      433           
  Lines        9430     9430           
  Branches     1020     1000   -20     
=======================================
  Hits         3370     3370           
- Misses       5376     5387   +11     
+ Partials      684      673   -11

Impacted Files	Coverage Δ
app/vue/src/server/utils.js	0% <0%> (-53.58%)	⬇️
addons/storyshots/src/rn/loader.js	53.33% <0%> (ø)	⬆️
addons/viewport/src/components/viewportInfo.js	36.36% <0%> (ø)	⬆️
lib/core/src/client/preview/story_store.js	84.21% <0%> (ø)	⬆️
...ddons/actions/src/containers/ActionLogger/index.js	49.25% <0%> (ø)	⬆️
addons/a11y/src/components/Tabs.js	0% <0%> (ø)	⬆️
addons/storyshots/src/react/renderShallowTree.js	57.14% <0%> (ø)	⬆️
addons/a11y/src/components/Report/RerunButton.js	0% <0%> (ø)	⬆️
app/angular/src/server/ts_config.js	40.9% <0%> (ø)	⬆️
addons/actions/src/lib/util/typeReviver.js	71.42% <0%> (ø)	⬆️
... and 51 more

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