Document the optional `special_redirect_strategy` in the README

I'm not entirely sure about copy for this - I copied the relevant bits of explanation from the story.
How about:

When the transition is partial, some tools or content will be left behind and managed by the previous supplier. This setting can be one of:
  * `via_aka` - the supplier is redirecting some paths to our `aka` domain
  * `supplier` - the supplier is managing redirects to gov.uk. No traffic comes through Bouncer for this site.