i2c-stub: Documentation update

The typical use-case is like this:

	1. load this module

	2. use i2cset (from lm_sensors project) to pre-load some data

	3. load the target sensors chip driver module

	2. use i2cset (from the i2c-tools project) to pre-load some data

	3. load the target chip driver module

	4. observe its behavior in the kernel log

There's a script named i2c-stub-from-dump in the i2c-tools package which