Change readme to explain the differences between firefish and ray

[dodoradio]

Currently, this repo says that it is for the skagen falster 2, rather than all Fossil gen 4 watches. It would be useful to add some information summarising the difference between the two builds (the display and multitouch drivers) and add a list of the Fossil gen 4 watches that fall into either category, with a label of whether they have been tested.
These are definitely fossil gen 4, there may be more:

Ray:
Skagen falster 2, tested
Fossil Q Venture HR
Misfit vapor 2 (41mm)

Firefish:
Fossil Q Explorist HR, tested
Diesel On Full Guard 2.5
Misfit vapor 2 (46mm)