Commit d449772cf6 stopped setting
COINBASE_FLAGS, and it looks like it hasn't been used since P2SH.
Update the help string to remove "flags", which is not specified in
BIP 22.
Github has started supporting SECURITY.md to contain a project's
security policy. Right now, the only place to find this project's
security contact is on bitcoincore.org. Adding this information to the
repository makes it easier to find as SECURITY.md becomes a standard.
This is copied almost exactly from https://bitcoincore.org/en/contact/
and based on conversations with Ethan Heilman.