documentation

added some documentation on customizing bda.plone.shop based on my experiences integrating the package in our customers plonesite.

i also added some installation instructions and a troubleshooting section since getting everything to work well currently means pinning some versions and even using source checkouts of yafowil (for proper translations) and collective.z3cform.datagridfield
we should make sure to get out releases of these packages when we tag/release our shop components