Add a security.txt file to torproject.org

security.txt files give people the information they need to contact Tor when they find a security issue.

It's an IETF draft, and Google has done it, so maybe we should too: https://securitytxt.org/

We can use the existing information at: https://www.torproject.org/about/contact#security

And we might want to:

• add a PGP key file
• add a signature
• maybe add a policy or acknowledgements when we decide how they work