fdd7fa19a docs: create security policy (Neha Narula)
Pull request description:
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 EthanHeilman.
ACKs for commit fdd7fa:
laanwj:
ACK fdd7fa19a9fdc6f15208280201e0ce186e35a9c8
Tree-SHA512: 9d6b93d10fff6e9c7a5cb6d8c1f0660623cd7a015abac7738f2aa9d141075456e71612b830eb5c707275529e2099fb41a44c531e29d821c9d2857d22241a91c3
