Update index.yml

[colins]

Add on-demand-service-broker-release

Checklist for submission:

• [x] LICENSE and NOTICE files are up to date
• [x] at least one final release is checked in on the default repo branch (there's a .final_builds folder)
• [x] of use to the general community and will be maintained
• [x] github repo must be public
• [x] bosh create-release must run successfully against all final releases (the blobstore needs to be public)
  • if not all final releases are valid, specify a min_version
• [x] a README explaining what the repo does
• [ ] [optional] deployment manifests/ directory contains example manifest