update readme

Author: nuclearace

README.md

@@ -92,6 +92,7 @@ Options
 - `nsp: String` Default is `"/"`
 - `cookies: [NSHTTPCookie]?` An array of NSHTTPCookies. Passed during the handshake. Default is nil.
 - `log: Bool` If `true` socket will log debug messages. Default is false.
+- `sessionDelegate: NSURLSessionDelegate` Sets an NSURLSessionDelegate for the underlying engine. Useful if you need to handle self-signed certs. Default is nil.
 
 Methods
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