Update custom reader documentation to mention coordinates and available datasets

[djhoese]

Custom reader documentation currently only mentions creating a gridded area definition, but should also mention creating a SwathDefinition via the coordinates parameter in the YAML.

Other things that could/should be added (I'll add more if I think of them):

• available_dataset_ids, at least a sentence or two about it and pointing to the more thorough API docs

[gerritholl]

It would also be helpful to point out explicitly that it's not mandatory to list all the available datasets in the yaml file; for a NetCDF file with lots of datasets, this just duplicates the self-documenting NetCDF header.

[djhoese]

Wow, I'm sorry @gerritholl. I really thought I added it to the custom reader documentation. Long overdue. I'll see if I can get to this later today.