fix: README should not be generated

[bcoe]

The README.md is custom enough in this repo that it seems as though we'd be doing back-flips to keep it up-to-date with the auto-generated README in synthtool.

Fixes: googleapis/synthtool#209

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #1657 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1657   +/-   ##
======================================
  Coverage    98.5%   98.5%           
======================================
  Files           2       2           
  Lines         267     267           
  Branches        2       2           
======================================
  Hits          263     263           
  Misses          4       4

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