Add missing copy of Security Team's public key

[felixc]

The Rust Security Policy page (https://www.rust-lang.org/security.html)
links to the team's public key in three ways: a copy embedded at the
bottom of the page, a link to the MIT keyserver, and a link to a hosted
copy (https://www.rust-lang.org/rust-security-team-key.gpg.ascii). This
last link is currently broken, because the referenced file is not
present in the repository.

The version added in this commit is copied from the embedded one.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[alexcrichton]

Thanks!