Add photo of HKE4 battery charger to docs

Also outline the reason for the cable-tied cable.

Change-Id: I4a01079073bb05cf5bc07adaeb52e13f6c1e70c9
Add photo of a battery to the docs

Change-Id: If3545a0648fcd622b563c8066eda40522c16013f
Explain why the iMAX B6 charging cable is fixed to the charger

Change-Id: I0b7fca5f6f3f95a0732f25873950df48025458b1