Update third party RPC implementations list (#4977)

I have added the xsrpcj RPC implementation for java

docs/third_party.md

@@ -129,6 +129,7 @@ GRPC (http://www.grpc.io/) is Google's RPC implementation for Protocol Buffers.
 * https://github.com/awakesecurity/gRPC-haskell (Haskell)
 * https://github.com/Yeolar/raster (C++)
 * https://github.com/jnordberg/wsrpc (JavaScript Node.js/Browser)
+* https://github.com/ppissias/xsrpcj (Java)
 
 ## Other Utilities