Create SECURITY.md

[eviljeff]

Part of https://mozilla-hub.atlassian.net/browse/AMOENG-2640

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.08%. Comparing base (a3c9d92) to head (06a5f1f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #195   +/-   ##
=======================================
  Coverage   91.08%   91.08%           
=======================================
  Files          27       27           
  Lines        2814     2814           
  Branches      180      180           
=======================================
  Hits         2563     2563           
  Misses        248      248           
  Partials        3        3           

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[chrstinalin]

(Same nit as mozilla/extension-workshop#2427, but I'm not attached to it)