Add a birdview-on-the-source-code section to the user manual

Add a birdview-on-the-source-code section to the user manual

In http://thread.gmane.org/gmane.comp.version-control.git/42479,
a birdview on the source code was requested.

J. Bruce Fields suggested that my reply should be included in the
user manual, and there was nothing of an outcry, so here it is,
not 2 months later.

It includes modifications as suggested by J. Bruce Fields, Karl
Hasselström and Daniel Barkalow.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>