Release v3.1-RC3

[richfab]

Release v3.1-RC3

1. Spec: update “TBD” release date in README.md
2. Spec: merge release/vX.X-RC into master in https://github.com/MobilityData/gbfs.
3. Spec: publish the draft release in Releases.
4. JSON Schema: merge release/vX.X-RC into master in https://github.com/MobilityData/gbfs-json-schema.
5. Validator: merge release/vX.X-RC into master in https://github.com/MobilityData/gbfs-validator.
6. Website: merge release/vX.X-RC into master in https://github.com/MobilityData/gbfs.org.

[richfab]

Done