Update readme with change to the way assembly variants work

Now you can not populate a part only in select assembly variants.
Merge branch 'feature-controller-update' of github.com:timburke/MoMo-Firmware into feature-controller-update
Re-layout the controller board for 1M form factor

Not done but mostly there.
Finish initial layout of revised controller board with enhanced pic

- now has battery backup of RTC
- new pic24 chip with 64K rom and 8K RAM
- lower power consumption
- removed current meter to battery for charging current (just measure
  the voltage over time)
Update board to latest version of eagle