Update README once "per-file detection" is merged and before release

[rlefevre]

#653 does not include README.md update.

Particularly this part:

You must run elm-format from the directory that contains your elm.json (for Elm 0.19) or elm-package.json (for Elm 0.18), or else you must pass the appropriate --elm-version=0.19/--elm-version=0.18 command line argument.

must be updated before the first release integrating the PR.