Notes for "getting started" guide

In answer to a question on Elm slack re getting started with elm-format, I thought this answer might be good to have in the official documentation:

> it’s recommended to enable “format on save” for whatever editors (the guideline for plugin authors is that hopefully it enables that by default)
>
> To initially format (or for folks who don’t want to format on save), `elm-format .` on the command line is recommended.
>
> And to enforce formatting on CI, `elm-format --validate `. is recommended.

Start in the official documentation's “getting started” guide and review how editor setup and CLI usage are currently described. Add guidance for format-on-save, initial formatting with `elm-format .`, and CI validation with `elm-format --validate`; done means these recommendations appear clearly in that guide.

Written by the indexing model from the issue text.