Document field description format

[skh]

Packages may contain fields definitions in YAML files in dataset/DATASETNAME/fields/ directories. These files are used by EPM to generate elasticsearch index templates and kibana index patterns. The format of these files comes from the beats project, but has never been documented.

This task is to document this format in this repository.

[mtojek]

I know that it's not a fully defined schema, but I've recently defined a structure for a single field (for serialization purposes): https://github.com/elastic/package-registry/blob/master/dev/import-beats/fields.go#L21

[ruflin]

@skh You might be able to copy some parts from here: elastic/beats#12505

[mtojek]

We have the package-spec now and these files are also covered. Resolving.