Help Discourse users remember where to report security concerns

[pshiu]

A security vulnerability was disclosed publicly on Discourse instead of privately via email.

We discovered this after someone helpfully sent us an email that such a post was made:

https://groups.google.com/a/openedx.org/g/security/c/JPTv43gvXo8

This issue is to add reminders on discuss.openedx.org of where to appropriately report security concerns or vulnerabilities.

[pshiu]

Ideas for where to put the reminders:

• Guidelines

• Welcome message:

[pshiu]

Next steps:

• Come up with text to add to these documents
• Propose the add to Axim