As I am no longer on the `security@bitcoincore.org` alias, there is no
point in asking people to GPG-encrypt to me. Remove it.
Tree-SHA512: f15640b741b908b162acf149ab27dae149fc917c15fbabf8dd9aff7bc6afb9e8b2ae956f9a0f8fbe1f05f40a48215becc1efda90a2e9c1105559cdeafb073060
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.