doc: add TLS session resumption example

Using TLS session resumption correctly is not obvious. This added
example code should help new users understand how to use it correctly.

Related issue: #3132
PR-URL: #3147
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>

doc/api/tls.markdown

@@ -603,6 +603,16 @@ perform lookup in external storage using given `sessionId`, and invoke
 NOTE: adding this event listener will have an effect only on connections
 established after addition of event listener.
 
+Here's an example for using TLS session resumption:
+
+  var tlsSessionStore = {};
+  server.on('newSession', function(id, data, cb) {
+    tlsSessionStore[id.toString('hex')] = data;
+    cb();
+  });
+  server.on('resumeSession', function(id, cb) {
+    cb(null, tlsSessionStore[id.toString('hex')] || null);
+  });
 
 ### Event: 'OCSPRequest'