Bug#940144: developers-reference: document self-service givebacks in wanna-build section

[Drew Parsons]

Has the self-service wannabuild giveback script been disabled?
It's now rejecting connections, e.g. 
https://buildd.debian.org/auth/giveback.cgi?pkg=ga&suite=sid&arch=armel 
generates

  Forbidden
  You don't have permission to access this resource.Reason: Cannot 
perform Post-Handshake Authentication.
  Apache Server at buildd.debian.org Port 443

My SSO is otherwise working fine, e.g. triggering debci tests at 
https://ci.debian.net/user