Update security policy.

[davidlehn]

• Link to policy in readme.
• Point at GitHub security system.
• Add suggestions for a helpful report.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 16.43%. Comparing base (7a43db9) to head (e1e4197).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1155   +/-   ##
=======================================
  Coverage   16.43%   16.43%           
=======================================
  Files          42       42           
  Lines        9597     9597           
=======================================
  Hits         1577     1577           
  Misses       8020     8020           

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