Explain how to disable vim-prettier default settings in readme

[ejoubaud]

I spent a while figuring out why vim-prettier was adding some default CLI options like --tab-width=4 to the prettier command it was running, which was causing changes that would break my CI build when it would check the codebase against a .prettierrc that (obviously) didn't include vim-prettier's default settings.

In case anyone encounters a similar issue, here's what I found out:

1. vim-prettier always adds a bunch of CLI flags with its own default settings to the prettier command it runs. See https://github.com/prettier/vim-prettier/blob/e122231/autoload/prettier/resolver/config.vim#L8
2. it even overrides some of its own default settings, like if g:prettier#config#tab_width is set to auto (the default), it will set it to the value of the shiftwidth vim setting (see https://github.com/prettier/vim-prettier/blob/e122231/autoload/prettier/resolver/config.vim#L56). In my case it would set it to 4 on markdown files

There doesn't seem to be any option to prevent vim-prettier from adding all those CLI flags, but thankfully we can ask prettier to ignore them whenever a config file is passed with the --config-precedence prefer-file option.

I thought this deserved a clearer mention in the section of the README that mentions the vim-prettier default config. WDYT?

(Thanks to @twm who put me on the right track in this issue comment: https://github.com/prettier/vim-prettier/issues/266#issuecomment-953417529)