message: Add SECURITY.md
  Summary:
  This is a backport of libsecp256k1 PR679
  Depends on D5362
  Test Plan:
  \o/
  Reviewers: #bitcoin_abc, jasonbcox
diff: @@ -76,3 +76,8 @@ Exhaustive tests
  With valgrind, you might need to increase the max stack size:
  $ valgrind --max-stackframe=2500000 ./exhaustive_tests
  +
  +Reporting a vulnerability
  +------------
  +
  +See [SECURITY.md](SECURITY.md)
  