chore: add SECURITY.md

[google-cloud-policy-bot]

chore: add SECURITY.md

[codecov]

Codecov Report

Merging #546 (2f35e31) into master (988fecf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #546   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          22       22           
  Lines       11952    11952           
=======================================
  Hits        11906    11906           
  Misses         46       46           

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