Update README.md

I struggled to get the section about using https to http with two certificates to work, because I didn't understand where the myCert etc. came from. Once I realised they had to be created by the user I could get it working. I want to save someone else the time it took me.

Author: richard-uk1

Diff for: README.md

@@ -344,6 +344,10 @@ var certs = {
 //
 // Proxy options
 //
+// This section assumes that myCert, myKey and myCa are defined (they are not
+// in this example). With a SNICallback, the proxy needs a default set of
+// certificates to use.
+//
 var options = {
   https: {
     SNICallback: function (hostname) {