Update the documentation for XNIO 3.x

The online documentation (especially the Developer Guide) is very outdated, and is for version 2.0.
More-over the documentation stops at section 4.4 with only empty section headers for the next.

This documentation is no longer valid for XNIO 3.0. For example the following code does not work:

TcpServer server = xnio.createTcpServer(openListener, OptionMap.EMPTY);
server.bind(new InetSocketAddress(12345));
server.bind(new InetSocketAddress(23456));

It does not work because createTcpServer() is no longer a method for xnio.

This documentation needs to be updated because the Developer Guide is the only quick start available for developers to know from where to start to get up and running with XNIO. The only documentation available is the Javadoc which only lists the classes without any indication of the order of how things should be invoked and what should be done to get things up and running.