Write down the security policy

[silkeh]

Write up and implement a policy for reporting security issues (this is currently undocumented).

Tasks:

• [x] Create security policy (see below)
• [x] Create [email protected] email address if needed.
• [x] Document in SECURITY.md (in this repo): #3991
• [ ] Add link to SECURITY.md to group README. https://github.com/getsolus/.github/pull/1
• [x] Document on website. https://github.com/getsolus/solus-site.github.io/pull/46
• [x] Document on Help Center https://github.com/getsolus/help-center-docs/pull/566