Add Security Policy

This patch adds a security policy stating how to handle and repost
security issues found in this library.

SECURITY.md

@@ -0,0 +1,17 @@
+Security Policy
+===============
+
+Supported Versions
+------------------
+
+Only the latest version of this library is supported.
+We are doing our best to make updates as easy as possible
+so that keeping up-to-date is usually pretty easy.
+
+
+Reporting a Vulnerability
+-------------------------
+
+If you find a security vulnerability,
+please report it by sending a mail to [email protected].
+We will discuss the problem internally and, if necessary, release a patched version as soon as possible.