Fixes #646
WIP because the secp256k1-security@bitcoincore.org email address doesn't exist yet. But it seems like the right place for vulnerability reports. security@bitcoincore.org would have the downside that it perhaps reaches more people than necessary. Ideally secp256k1-security would just forward to the three maintainers listed in SECURITY.md. @sipa @apoelstra is it okay to put you there? Fwiw I'm opting out for now because three people should be enough. @sipa do you know who to talk to about adding secp256k1-security@bitcoincore.org and the specifics about how it would work?