README for rationale behind DeDrift... options

[coalsont]

I think a description of all this as part of a README would be very helpful for users attempting to understand dedrifting and how it relates to MSMAll. I realize that said README might have to be a little bit messy, perhaps with both "optimal implementation" and "actual/current implementation" components, but that's the reality of the current situation.

Originally posted by @mharms in #182 (comment)

The current MSMAll implementation as used in all HCP projects expects users that want to match it to use "dedrifting" arguments, in a way that isn't truly dedrifting. When we come up with a more refined registration, we can change the procedure used such that these arguments are no longer required for end users. In the meantime, we should document the current situation.