Revamp Plan: v3 and v4

Nobody has claimed this yet.

👋 We are ramping up efforts to document the current spec as well as discuss a future version with all contributors. This ticket serves as the overarching plan forward. @mapsam and I are excited to be leading out this process and collaborating with you all.

### What’s been happening?
No doubt there is a need to have a clear set of tileset metadata in order to support interoperability between different tile-based geo tools. This is the need TileJSON aims to fill. However, the TileJSON spec has not been consistently maintained for a number of years due to original owners moving on to other projects or organizations, which has led to inconsistencies. @mapsam and I are picking up the torch to address this.

We are committed to ensuring the current spec as well as future spec additions are properly documented and all changes fully transparent. To achieve this, all proposed changes will follow TileJSON spec’s [contributing procedures](https://github.com/mapbox/tilejson-spec/blob/master/CONTRIBUTING.md). @mapsam and I are also committed to shepherding TileJSON spec’s [Code of Conduct](https://github.com/mapbox/tilejson-spec/blob/master/CODE_OF_CONDUCT.md) during this ramp up. Please reach out to either of us if you have any questions about these docs.

## Phase 1

```
v3.0
```

**Goal**: Solidify our understanding of the current spec, documenting, then release `v3.0`. This will enable us all to discuss and implement our visions for how we want things to be in Phase 2 (see below).

Phase 1 will center around fully documenting the already-existing spec as well as any relevant historical context. We are prioritizing this due to there being an ownership gap and a general lack of clarity in the past around fast changes and maintenance of the spec. We’ll plan to create PRs for each of the next actions below, where we will continue discussion for each.

We are aiming to finish Phase 1 by May 2018.

### Next Actions

- Add undocumented properties and expand on documented properties as needed
- Respond to all existing issues regarding Phase 1, adding historical context as needed
- Reformat and improve searchability of README
- Ensure consistency with [RFC 2119](https://www.ietf.org/rfc/rfc2119.txt)
- Include concrete [examples](https://github.com/mapbox/tilejson-spec/blob/master/2.2.0/example/osm.layer) for every field
- Create Milestone for easy tracking

## Phase 2

```
v4.0
```

**Goal**: Discuss modifying current properties and adding new features, then release `v4.0`

### Next Actions

- Respond to all existing Issues proposing new additions, changes, or removals
- Consider how `v4.0` is impacted by the [upcoming Vector Tile 3 spec](https://github.com/mapbox/vector-tile-spec/issues/103)
- Create Milestone for easy tracking

Thanks for your continued commitment! 🙌

cc @mapbox/core-tech

This issue is an overarching v3/v4 roadmap rather than a single change. Start with the README, existing issues, and the 2.2.0/example/osm.layer example; review undocumented properties, RFC 2119 consistency, and the proposed documentation work. Done would require coordinated updates, examples for every field, and tracked milestones for the releases.

Written by the indexing model from the issue text.