Add security policy

[JakeHartnell]

Going to have some people going through the contracts and looking for bugs. I don't see a security policy yet and it would great to have one so folks can follow the process if they find something.

CC @elsehow, who helped update Juno's recently.

[ethanfrey]

We got one now!
https://github.com/CosmWasm/cw-plus/blob/main/SECURITY.md