Added comments from readme

Added comments from readme at
http://www.dr-qubit.org/undo-tree/undo-tree.txt
describing the commands available in visualization mode and
visualization/selection mode.  These should guide the creation
of online help either within the temporary undo-tree frame or in the
minibuffer.
Merge branch 'help-in-visualizer-frame' of https://github.com/apchamberlain/undo-tree.el into help-in-visualizer-frame