Bufferline is outdated: Readme is incorrect

[thijssmudde]

The configuration of the readme is outdated regarding bufferline package. Please use a specific version of bufferline and/or update the Readme.

Using `gui`, `guifg`, `guibg`, `guisp` is deprecated please, convert these as follows:
- guifg -> fg
- guibg -> bg
- guisp -> sp
- gui -> underline = true, undercurl = true, italic = true
 see :help bufferline-highlights for more details on how to update your highlights
Please fix:
- buffer_visible