Readme: add note about configuring autoformat when config is present

[jonpacker]

Summary

Adds a simple note to the autoformat_config_present section documenting that the setting does nothing without switching off autoformat_require_pragma first. I found this confusing when setting up this plugin myself, and the reactions in #191 seem to indicate I wasn't alone 😅

[TimDaub]

Actually, I think this comment https://github.com/prettier/vim-prettier/issues/191#issuecomment-614280489 is instructive and should also be added (with slight modification) to the readme.md.

[jtheoof]

I was configuring vim-prettier today and run into the issue until I saw the link posted by @TimDaub and found this MR.

Really feel like this MR should be merged but I agree it would be even better with the comment posted.