Add security.txt

[0xTim]

The security.txt file is becoming a well known standard for finding security contact information for sites and IMO it would be a good idea for swift.org to adopt this as well to make it easy for researchers to reach out if they find any issues on the site or the language itself.

[parispittman]

With the move to /swiftlang, we have this pretty security boost on the tabs now that links to swift.org/support/security.html

What do you think?

[0xTim]

If we can add a security.txt to link to this security section on GH that would be a useful link otherwise anyone on the site would have to find where the GH repo is etc, which could be a slowdown for someone trying to report a security vulnerability in Swift or associated projects