Releasing new versions of the spec

[tomwhite]

So far we have been updating the spec at https://github.com/sgkit-dev/vcf-zarr-spec/blob/main/vcf_zarr_spec.md and bumping the version number as we go. However, this won't work if we want to make a series of changes and then release those changes in one go.

The simplest way to do this would be to make the changes in a branch then merge into main when we want to release a new version.

Thoughts?

[jeromekelleher]

That SGTM. Keeping things lightweight is a good goal here.

[tomwhite]

I created a 0.3 branch for this.