Add SECURITY.md

[zidingz]

Hey there!

I belong to an open source security research community, and a member (@lfama) has found an issue, but doesn’t know the best way to disclose it.

If not a hassle, might you kindly add a SECURITY.md file with an email, or another contact method? GitHub recommends this best practice to ensure security issues are responsibly disclosed, and it would serve as a simple instruction for security researchers in the future.

Thank you for your consideration, and I look forward to hearing from you!

(cc @huntr-helper)

[Shu12388y]

@zidingz can you assign this issue to me

[psmoros]

Hey there! I work with Zi, are you a contributor of express cart?