Clarify readme and fix typos.
Clarify readme and fix typos.

Fixes a few typos and clarifies a few lines. Also moves the MotorStepper
paragraph down a bit so that the explanation doesn't start with that.

iwanders/focus_stacking,iwanders/focus_stacking,iwanders/focus_stacking,iwanders/focus_stacking,iwanders/focus_stacking,iwanders/focus_stacking