chore: add SECURITY.md

[google-cloud-policy-bot[bot]]

chore: add SECURITY.md

[codecov[bot]]

Codecov Report

Merging #243 (c4b2787) into master (5678191) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #243   +/-   ##
=======================================
  Coverage   79.64%   79.64%           
=======================================
  Files           3        3           
  Lines         113      113           
  Branches       14       13    -1     
=======================================
  Hits           90       90           
  Misses         23       23           

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