patman: Add basic documentation for new features

patman: Add basic documentation for new features

Describe the new subcommands and how they should be used in a normal
workflow.

Note that the naming of branches is very rigid, or 'opinionated' in
marketing terms. Patman can track a single branch for each version of a
series and they must all be named the same, except for the
version-number suffix. Version 1 series have no suffix.

This description is fairly bare-bones but should be enough for some
initial testing and comments.

Signed-off-by: Simon Glass <sjg@chromium.org>