Draft guidance on selecting a transformation partner

Derived from earlier pull request, which was itself in part a
bringing together of other internal documents, and then added my
own comments.
Expand abbreviation
Remove thing

Too many things in that sentence!
Use obvious noun
Include contributions from Josh and Brad
Remove full stops from eg

Style guide FTW.
Merge pull request #88 from alphagov/selecting-partners

Draft guidance on selecting a transformation partner