Project: Adds a security policy (#17698)

Adds a security policy document. For reference, https://help.github.com/en/articles/adding-a-security-policy-to-your-repository.

Closes #17649

SECURITY.md

+Reporting Security Issues
+------------------
+
+If you think you have found a security vulnerability, please send a report to [security@grafana.com](mailto:security@grafana.com). This address can be used for all of Grafana Labs's open source and commercial products (including but not limited to Grafana, Grafana Cloud, Grafana Enterprise, and grafana.com). We can accept only vulnerability reports at this address. We would prefer that you encrypt your message to us; please use our PGP key. The key fingerprint is:
+
+F988 7BEA 027A 049F AE8E  5CAA D125 8932 BE24 C5CA
+
+The key is available from [pgp.mit.edu](https://pgp.mit.edu/pks/lookup?op=get&search=0xF9887BEA027A049FAE8E5CAAD1258932BE24C5CA) by searching for [grafana](https://pgp.mit.edu/pks/lookup?search=grafana&op=index).
+
+Grafana Labs will send you a response indicating the next steps in handling your report. After the initial reply to your report, the security team will keep you informed of the progress towards a fix and full announcement, and may ask for additional information or guidance.
+
+**Important:** We ask you to not disclose the vulnerability before it have been fixed and announced, unless you have got a response from the Grafana Labs security team that you can do that.
+
+### Security Announcements
+
+We maintain a category on the community site called [Security Announcements](https://community.grafana.com/c/security-announcements),
+where we will post a summary, remediation, and mitigation details for any patch containing security fixes. You can also subscribe to email updates to this category if you have a grafana.com account and sign on to the community site or track updates via an [RSS feed](https://community.grafana.com/c/security-announcements.rss).