Provide WIKI documentaion for module

[stpaultim]

I've enabled the github wiki for this module and begin to create some very simple documentation.
https://github.com/backdrop-contrib/d2b_migrate/wiki

[stpaultim]

@irinaz and @docwilmot - I'd welcome some feedback with what I've started to the Wiki. I've decided to not let perfection be the enemy of the good and just start collecting informtion there that I would find helpful. I would appreciate any feedback or suggestions on format, content, or structure?

[irinaz]

Looks great to me, I will be adding more if I see any other issues.