Added a security policy (netty#10692)

Motivation:

The process of reporting security issues should be documented
and easy to find.

Modification:

Added a SECURITY.md file that describes how to report a security issue.

Result:

It's a bit easier to find the docs that describe how security issues
should be reported. Also, when someone creates an issue the repository,
they will see a link to the security policy.

Author: artem-smotrakov

SECURITY.md

@@ -0,0 +1,5 @@
+# Reporting a security issue
+
+If you think the bug you found is likely to make Netty-based applications vulnerable to an attack,
+please do not use our public issue tracker
+but report it to [the dedicated private Google Group](https://groups.google.com/d/forum/netty-security).