README: Add echo server and client section

README.md

@@ -58,3 +58,37 @@ Actually at first I wanted to avoid `std::string` because they incur unneeded bu
 
 That initial implementation remains available in branch `string`.
 
+
+## Echo Server and Client
+
+Using [Beej's Guide to Network Programming](https://beej.us/guide/bgnet/html//index.html) as a handy reference, I wrote adapters for needed parts of the POSIX socket API. As much as practical, these adapters mirror [Python's socket API](https://docs.python.org/3/library/socket.html).
+
+These adapters include the functions:
+- `getaddrinfo`
+- `socket`
+- `setsockopt`	(only for options with integer values, which is all I needed)
+- `connect`
+- `bind`
+- `listen`
+- `accept`
+- `recv`
+- `send`
+- `shutdown`
+- `close`
+
+The new GIL-free strings proved very useful for this :)
+
+Since currently Cython+ doesn't have GIL-free exceptions (also in our next steps), I take the GIL and raise Python exceptions instead when there is an error.
+
+It's not complete, but it's enough to write a simple echo server and client demo:
+
+```
+make echoserver
+```
+
+and
+
+```
+make client
+```
+