docs: Added release notes and associated media for 0.38

[garden-of-delete]

SUMMARY

We would like to publish release notes with each major release. This is a first crack at sorting out the format for the release notes, starting with v0.38. These are the decision points that would benefit from feedback or objections, and will likely see iteration in future release notes:

• Multimedia embedded in the release notes vs no multimedia
• A single .md file with each new version's notes appended vs a separate .md file and directory for each release
• Directory structure for release notes
• How the release notes are linked to the top level README.md

The changes in this PR make a suggestion for each of those points.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TEST PLAN

No testing required, no changes to codebase.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #12022 (153f124) into master (b75a1ec) will increase coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12022      +/-   ##
==========================================
+ Coverage   66.91%   67.15%   +0.23%     
==========================================
  Files         994     1002       +8     
  Lines       49095    49231     +136     
  Branches     4983     5000      +17     
==========================================
+ Hits        32852    33059     +207     
+ Misses      16118    16049      -69     
+ Partials      125      123       -2     

Flag	Coverage Δ
cypress	51.41% <ø> (+2.58%)	⬆️
javascript	61.25% <ø> (+0.04%)	⬆️
python	64.15% <ø> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...frontend/src/explore/components/OptionControls.tsx	75.60% <0.00%> (-24.40%)	⬇️
superset/db_engine_specs/mysql.py	79.59% <0.00%> (-12.25%)	⬇️
...explore/components/controls/AdhocFilterControl.jsx	65.64% <0.00%> (-2.90%)	⬇️
superset/cli.py	33.58% <0.00%> (-0.92%)	⬇️
superset/models/core.py	88.04% <0.00%> (-0.82%)	⬇️
...et-frontend/src/explore/reducers/exploreReducer.js	43.10% <0.00%> (-0.76%)	⬇️
superset/views/core.py	74.92% <0.00%> (-0.46%)	⬇️
superset/reports/dao.py	79.24% <0.00%> (ø)
superset/models/reports.py	100.00% <0.00%> (ø)
superset/async_events/api.py	100.00% <0.00%> (ø)
... and 57 more

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[benceorlai]

Thank Robert!

[mistercrunch]

See it rendered here:
https://github.com/garden-of-delete/incubator-superset/blob/master/RELEASING/release-notes-0-38/release-notes-0-38.md

Comments:

• file could be /release-notes-0-38/README.md so that github renders it when clicking on the folder
• expecting to see images embedded in the release notes but I don't see any (?)

[garden-of-delete]

See it rendered here:
https://github.com/garden-of-delete/incubator-superset/blob/master/RELEASING/release-notes-0-38/release-notes-0-38.md

Comments:

• file could be /release-notes-0-38/README.md so that github renders it when clicking on the folder
• expecting to see images embedded in the release notes but I don't see any (?)

Addressed in most recent commits. New location:
https://github.com/garden-of-delete/incubator-superset/blob/master/RELEASING/release-notes-0-38/README.md

[benceorlai]

@garden-of-delete Github's MD supports html tags, which gives you better control over the layout of images. Could you try something like:

<img src="/media/10800.gif" style="max-width:50%; border:1px solid grey;">

[etr2460]

spelling nit: changelog

[mistercrunch]

Super nit-picky, but can we add borders to images? https://stackoverflow.com/questions/37349314/is-it-possible-to-add-border-to-image-in-github-markdown

Looks a bit rough here:
https://github.com/garden-of-delete/incubator-superset/tree/master/RELEASING/release-notes-0-38/

[mistercrunch]

Trying the old close/reopen trick since Cypress is not complying

[robdiciuccio]

Nit: would love to see alt tags on the images for improved accessibility.