Add tests for examples from Nim in Action.

tests/niminaction/Chapter3/ChatApp/readme.markdown

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+# The ChatApp source code
+
+This directory contains the ChatApp project, which is the project that is
+created as part of Chapter 3 of the Nim in Action book.
+
+To compile run:
+
+```
+nim c src/client
+nim c src/server
+```
+
+You can then run the ``server`` in one terminal by executing ``./src/server``.
+
+After doing so you can execute multiple clients in different terminals and have
+them communicate via the server.
+
+To execute a client, make sure to specify the server address and user name
+on the command line:
+
+```bash
+./src/client localhost Peter
+```
+
+You should then be able to start typing in messages and sending them
+by pressing the Enter key.