Security: Generate and document GPG key(s)

**Has this feature been discussed and generally agreed?**

jamulussoftware/jamuluswebsite#319 was about documenting the process and a contact address for reporting security issues. It was also discussed to provide a way to contact the project in encrypted form. This still needs to be done.

I'm opening this on the code repo as this is where `SECURITY.md` lives and will have to be updated.

**Describe the solution you'd like**

Variants:
1. Document each developer's own GPG key (and address) on their team page (tbd)
2. Generate a key for team@ and share it.
Both variants have advantages and disadvantages, see https://github.com/jamulussoftware/jamuluswebsite/issues/319#issuecomment-803663521 and later comments.