adding in readme for @keyv/compress-brotli

[jaredwray]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/jaredwray/keyv/blob/main/CODE_OF_CONDUCT.md
• Tests for the changes have been added (for bug fixes/features) with 100% code coverage.
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  adding in readme for @keyv/compress-brotli

[codecov]

Codecov Report

Merging #453 (fe4326a) into main (b14b372) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #453   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines         1103      1103           
=========================================
  Hits          1103      1103           

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