docs: add custom readme content

[mctavish]

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Fixes #1086 🦕

[codecov[bot]]

Codecov Report

Merging #1100 (5c4e4e4) into main (bbaa42e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1100   +/-   ##
=======================================
  Coverage   65.79%   65.79%           
=======================================
  Files          21       21           
  Lines        1751     1751           
  Branches      352      352           
=======================================
  Hits         1152     1152           
  Misses        512      512           
  Partials       87       87           

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[generated-files-bot[bot]]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml
• samples/README.md - samples/README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml