Documentation updates, minor menu change

This update includes a very rough draft documenting the menu
configuration.  The README now includes a description of why the
automatic drift correction is not enabled immediately.

Also the automatic dimmer maximum and minimum labels have been changed
from "b min XX" and "b max XX" to "b. min XX" and "b. max XX".
Minor drift correction bugfixes

Two minor drift correction bugfixes should (very) slightly improve the
accuracy of the automatic drift correction.