Added note about standard library module xdrlib.py

Demo/rpc/README

 This is a Python interface to Sun RPC, designed and implemented mostly
 by reading the Internet RFCs about the subject.
 
+*** NOTE: xdr.py has evolved into the standard module xdrlib.py ***
+
 There are two library modules, xdr.py and rpc.py, and several example
 clients: mountclient.py, nfsclient.py, and rnusersclient.py,
 implementing the NFS Mount protocol, (part of) the NFS protocol, and