Add how users should report security vulnerabilities for this repository (#2562)

## Proposed Changes

Suggestion to add a notice on how to report security vulnerabilities. This is visible at https://github.com/sigp/lighthouse/security

Author: fredriksvantes

SECURITY.md

@@ -0,0 +1,13 @@
+# Security Policy
+
+## Supported Versions
+
+Please see [Releases](https://github.com/sigp/lighthouse/releases/). We recommend using the [most recently released version](https://github.com/sigp/lighthouse/releases/latest).
+
+## Reporting a Vulnerability
+
+Please send vulnerability reports to [email protected] and encrypt sensitive messages using our [PGP
+key](https://keybase.io/sigp/pgp_keys.asc?fingerprint=15e66d941f697e28f49381f426416dc3f30674b0).
+
+**Please do not file a public ticket** mentioning the vulnerability, as doing so could increase the likelihood of the vulnerability being used before a fix has been created, released and installed on the network.
+