Add semver document

[nvkelso]

As we approach v1.0.0 of Tilezen, what promises are we making? When should we bump the version on future changes?

Developers should be able to tell from the Tilezen version increment how much effort it will take them to integrate the new tiles with their map. We use semantic versioning to communicate this.

Add documentation for how Tilezen proposes to implement SemVer for vector tiles.