Synchronize common files from prometheus/prometheus (#793)

* Update common Prometheus files

Signed-off-by: prombot <prometheus-team@googlegroups.com>

* Ignore Prometheus markdown from linter.

Ignore synced CODE_OF_CONDUCT and SECURITY files from linter.

Signed-off-by: Ben Kochie <superq@gmail.com>

Co-authored-by: Ben Kochie <superq@gmail.com>

.markdownlintignore

@@ -0,0 +1,2 @@
+CODE_OF_CONDUCT.md
+SECURITY.md

CODE_OF_CONDUCT.md

@@ -1,3 +1,3 @@
-# Community Code of Conduct
+## Prometheus Community Code of Conduct
 
 Prometheus follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).

SECURITY.md

@@ -0,0 +1,6 @@
+# Reporting a security issue
+
+The Prometheus security policy, including how to report vulnerabilities, can be
+found here:
+
+https://prometheus.io/docs/operating/security/