docs: Propose a security policy

[trishankatdatadog]

Propose a security policy (largely borrowed from go-tuf) that users should consult in order to report any security vulnerability.

Note that privately reporting security vulnerabilities requires turning on a GitHub setting.

[trishankatdatadog]

Note that privately reporting security vulnerabilities requires turning on a GitHub setting.

Please don't forget the enable this setting 🙂

[ianlewis]

Please don't forget the enable this setting 🙂

The setting is enabled.