Clarify in README and documentation how DGGS is supported

[benbovy]

We should clearly mention in the README and the documentation how DGGS are supported here. This would also help in better define the scope of this library. Some aspects below (I might miss other important aspects).

Full global coverage vs. partial vs. "sparse" cell coverage

I think we should support both.

• Example of full coverage (source)

• Example of partial and sparse coverage (source)

Multi-resolution vs Fixed-resolution

Currently we only support fixed resolution.

• Example of multi-resolution (source)

• Example of fixed-resolution (source)

Coordinate array dimensions

Currently we only support 1-d coordinates (like xvec).