Add spec for register metadata

[lossyrob]

Currently the register has JSON entries that point to contributions, but it's not clear what data is necessary for someone to contribute to OIN. This is especially true for contributions in storage types different than S3. We should create a document in this repository that specifies how the register metadata should be formatted and what data needs to be included.

[mojodna]

#2 makes me think that each location entry should include an optional capabilities entry that describes whether range requests (etc.) are supported.