Add documentation for querying a Dataset

[tbarn]

In the 3.7.0 release, we added querying support for Datasets. You can find the code in the #233 PR.

KeenClient-iOS/KeenClient/KeenClient.h

Line 413 in 3d467eb

Gets results from a cached dataset query, which can be a single or multi-analysis query.

Like the comments in the code say, you can find detailed API documentation for it here: https://keen.io/docs/api/?shell#retrieving-results-from-a-cached-dataset
And here: https://keen.io/docs/compute/cached-datasets/

It would be nice to have an example for it in the README.md though.

The JavaScript section of the API reference is a good example of these docs: https://keen.io/docs/api/?javascript#retrieving-results-from-a-cached-dataset

Let me know if you have any questions!