Add README.

Up to now the API of gsu was completely undocumented. This commit
tries to improve on this. It adds a README file which explains the
public functions and variables of the three gsu modules (subcommand,
config and gui). Most features are illustrated by examples.

The README file is formatted in the grutatxt markup language, so it
can easily be converted to a nice html document.

Many thanks to George Wang who read an early draft of this document
and contributed many improvements.