added UDP to socket 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.
+This mimimal example consists of two pars of files for TCP and UDP:
+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. (In the UDP case, the client sends a
+datagram and the server echoes it back, as there is no connection
+established, so the "client" must start by sending a datagram.
+Note that the terms "client" and "server" in the UDP case have a
+more vague meaning, where the "client" is the one that takes the initiative
+by sending the first datagram.)
 
 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,
+An alternative to using simple_tcp_client is to use the telnet program to
+connect to the server. To do that, start `simple_tcp_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.
\ No newline at end of file
+Sometimes, `telnet` is a useful tool to look at or debug text-based protocols
+over TCP.
+
+
+Please note that TCP is connection-based, but not UDP. Thus, when using
+UDP, there is nothing like `listen()` or `accept()`. Instead, you simply
+send datagrams using `sendto()` or `sendmsg()`, and receive datagrams
+with `recvfrom()` or `recvmsg()`. See the manual pages for further information.
+