created socket example readme

socket_examples/README-socket.md

+# Introduction to socket programming in C
+
+This mimimal example consists of two files, a server which replies with
+a message to the first client that connects and then exits, and a client
+which connects to a server and outputs whatever it receives.
+
+See "Socket programming in C" for more details.
+
+To build and run the example, do `make run`. See the Makefile for details
+
+An alternative to using simple_client is to use the telnet program to
+connect to the server. To do that, start `simple_server` in one terminal,
+and then type `telnet localhost 5000` in another.
+
+Sometimes, `telnet` is a useful tool to look at or debug text-based protocols.
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