Add README to describe the STAC domain model

[lossyrob]

We need to generate some text for the README that describes the domain model of the STAC at a high level. Some components to be defined are:

• SpatioTemporal Asset Catalog (STAC): This is the general name of the catalog system specified in this repository.
• Item: An Item is contained in a STAC, and is described by a OGC SimpleFeature (expressible as GeoJson) that contains the footprint and timestamps of one or more linked Items or Assets.
• Asset: An Asset represents the final thing a Catalog can point to, such as a GeoTiff representing imagery, a laz file representing some lidar information, a metadata file describing other assets, etc.

We should potentially also include the concept for a STAC that was introduced by the Static Catalog working group in the Boulder Sprint, namely the idea that a STAC JSON representation can point to other STACs, creating a tree of crawlable, hierarchically grouped STACs.

[lossyrob]

Working with https://github.com/radiantearth/stac-spec/blob/feature/add-flat-file-spec/spec/json/README.md. @notthatbreezy will iterate on this over implementing a full NAIP example, and work on writing up the open api spec.

[cholmes]

This is outdated, and I think the core has been covered.