Migrate storage spec

[alimanfoo]

Assuming we want to at least migrate the storage spec versions 1 and 2, then:

• Migrate the source files for the storage spec from the zarr (python) repo to this repo.
• Set up RTFD for this repo and (re)publish the specs from here (I guess they'd end up at zarr-specs.readthedocs.io).
• Edit the docs back at zarr.readthedocs.io to explain that specs have moved and provide links to new locations.

[joshmoore]

With GitHub beta enabled, owners of a repository should also be able to transfer issues between repositories:

for the likes of 280, 276, perhaps 317, 388, 389, etc.

[jakirkham]

Sounds good. Was waiting until we resolved the naming and licensing issues.