docs: create security policy
docs: create security policy

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.


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