SECURITY.md: update with current practices

Daiki Uenorequested to merge
dueno/gnutls:wip/dueno/security-policy into master
  • SECURITY.md: update with current practices

Fixes: #1881 (closed)

Checklist

  • Commits have Signed-off-by: with name/author being identical to the commit author
  • Code modified for feature
  • Test suite updated with functionality tests
  • Test suite updated with negative tests
  • Documentation updated / NEWS entry present (for non-trivial changes)

Reviewer's checklist:

  • Any issues marked for closing are addressed
  • There is a test suite reasonably covering new functionality or modifications
  • Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
  • This feature/change has adequate documentation added
  • No obvious mistakes in the code

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