Add current development JSON schema

[letmaik]

See covjson/covjson-validator#32 (comment) for context.

[jonblower]

A couple of questions (in my ignorance of how these things are done):

1. Should we make the schema compact (removing unnecessary whitespace?)
2. Does the schema need to declare which version of JSON-Schema it supports?

[letmaik]

A couple of questions (in my ignorance of how these things are done):

1. Should we make the schema compact (removing unnecessary whitespace?)

I don't think so. If anyone has a need for it, they can compact it themselves, but typically schemas are not published compacted.

2. Does the schema need to declare which version of JSON-Schema it supports?

Yes, it does actually, in line 80. Should probably change the code to make sure this is at the top to avoid confusion.

[jonblower]

OK, that's all fine with me then. If it's quick to change the code to put the schema version at the top, I suggest doing that, but no worries if it's a pain.

I'll leave you to squash and merge - take this as unofficial PR approval!