Add SECURITY.md file (#8292)

* Add SECURITY.md file
Signed-off-by: NJulien Pivotto <roidelapluie@inuits.eu>

SECURITY.md

+# Reporting a security issue
+
+The Prometheus security policy, including how to report vulnerabilities, can be
+found here:
+
+https://prometheus.io/docs/operating/security/

scripts/sync_repo_files.sh

@@ -20,7 +20,7 @@ if [ -z "${GITHUB_TOKEN}" ]; then
 fi
 
 # List of files that should be synced.
-SYNC_FILES="CODE_OF_CONDUCT.md LICENSE Makefile.common"
+SYNC_FILES="CODE_OF_CONDUCT.md LICENSE Makefile.common SECURITY.md"
 
 # Go to the root of the repo
 cd "$(git rev-parse --show-cdup)" || exit 1