update READMEs

[handeyeco]

Summary:

I was looking for READMEs to help me understand what packages do.

Some of them didn't have READMEs.

I don't know what they do, so it's hard for me to write READMEs.

Input appreciated.

[khan-actions-bot]

Gerald

Required Reviewers

• @Khan/perseus for changes to README.md, .changeset/giant-pants-build.md, packages/kas/README.md, packages/kmath/README.md, packages/math-input/README.md, packages/perseus-error/README.md, packages/perseus-linter/README.md, packages/pure-markdown/README.md, packages/simple-markdown/README.md

---

Don't want to be involved in this pull request? Comment #removeme and we won't notify you of further changes.

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (9118af8) and published it to npm. You
can install it using the tag PR1407.

Example:

yarn add @khanacademy/perseus@PR1407

If you are working in Khan Academy's webapp, you can run:

./dev/tools/bump_perseus_version.sh -t PR1407

[github-actions]

Size Change: 0 B

Total Size: 854 kB

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	38.2 kB
packages/kmath/dist/es/index.js	4.26 kB
packages/math-input/dist/es/index.js	80.8 kB
packages/math-input/dist/es/strings.js	1.73 kB
packages/perseus-core/dist/es/index.js	906 B
packages/perseus-editor/dist/es/index.js	272 kB
packages/perseus-error/dist/es/index.js	866 B
packages/perseus-linter/dist/es/index.js	21.8 kB
packages/perseus/dist/es/index.js	414 kB
packages/perseus/dist/es/strings.js	3.21 kB
packages/pure-markdown/dist/es/index.js	3.67 kB
packages/simple-markdown/dist/es/index.js	12.4 kB

_{compressed-size-action}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.00%. Comparing base (147ab04) to head (9118af8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1407      +/-   ##
==========================================
+ Coverage   69.71%   71.00%   +1.29%     
==========================================
  Files         505      509       +4     
  Lines      105131   105217      +86     
  Branches     7590    11347    +3757     
==========================================
+ Hits        73293    74714    +1421     
+ Misses      31662    30503    -1159     
+ Partials      176        0     -176     

see 148 files with indirect coverage changes

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[jeremywiebe]

Nice! Thank-you!

[catandthemachines]

+1 Looks like a great!