Missing security policy

[AndriiLab]

Hi riok team,

Is it possible to define the security policy in https://github.com/riok/mapperly/security ?
I'd like to use your solution, but I got rejected by our security team because there is no formal process for reporting vulnerabilities or handling security issues

[latonz]

Security policy added.

[AndriiLab]

Thank you @latonz !