Add security policy (SECURITY.md)

[Xcatolin]

Add a security policy to the project so that security-related issues can be informed and tracked. Example below.

• https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository

Security Policy

Supported Versions

Currently, the following versions of Numbas is supported.

Version	Supported
7.x	:white_check_mark:
< 6.x	:x:

Reporting a Vulnerability

Potential security vulnerabilities can be reported directly to us at email@example.com. The team will provide a feedback after the case is analyzed.

Please make sure to include enough information so that the issue can be reproduced on our side.

[christianp]

Thanks, I've done that.