Updated read me, added self-contained initialization

This just adds a few additional comments in the README, and replaces the AHRS based initialization with a sensor-based init.

I believe that this is beneficial in terms of generality (no need for a piggyback AHRS log) and also in terms of comparison: When comparing the attitude estimate here vs. other approaches, both should be self contained and the initialization (and convergence) is a valid aspect of the comparison.