Enhance `GET /kinds/{kind-id}` with a markdown field or something for signup hints, etc.

[garyb]

So we can do things like this https://github.com/precog/precog-frontend/issues/111, for the form page at least. I'd strongly prefer not to put that sort of thing on the destination kind listing page in the real product.

[garyb]

Or maybe not, seems like we're only going to be advertising Snowflake.