Document field name conventions

It would be nice to include guidelines about the conventions for field naming in the [field `name` documentation](https://github.com/elastic/package-spec/blob/b366871e9da864871f9300dc4c8e379739e72e10/spec/integration/data_stream/fields/fields.spec.yml#L21).
And/or we can also consider adding documentation about this to [`elastic-package` howtos](https://github.com/elastic/elastic-package/blob/ba7e6801f6944617e5ca33ebd60402385bf079fc/docs/howto).

Current recommendations are assumed to be the same as for beats ([here](https://www.elastic.co/guide/en/beats/devguide/current/event-conventions.html)), but it'd be good to review and document them in the current context of Elastic Packages and ECS.