Add a security policy

[psmoros]

Hello 👋

I run a security community that finds and fixes vulnerabilities in OSS. A researcher (@h2oa) has found a potential issue, which I would be eager to share with you.

Could you add a SECURITY.md file with an e-mail address for me to send further details to? GitHub recommends a security policy to ensure issues are responsibly disclosed, and it would help direct researchers in the future.

Looking forward to hearing from you 👍

(cc @huntr-helper)

[Piyush-Bhor]

They have a bug bounty program on Huntr.com. Here's the link: https://huntr.com/bounties?target=https://github.com/nose-devs/nose