Add 'developers in the team' section

This replaces the 'GDS will provide guidance' section with something
more useful and that doesn't necessarily commit us to more work before
we've got more to say.
Rephrease slightly

To address concerns from @fatbusinessman and @philandstuff
Merge pull request #84 from alphagov/remove-timeliness-from-dev-section

Add 'developers in the team' section