Disable linting autocmd for readonly buffers

[ccjmne]

Disable linting autocmd for readonly buffers.

This should avoid linting in buffers outside of the user's control, having in mind especially the handy LSP pop-ups that describe your hovered symbol using markdown.

I added @rgruyters as a co-author, because he's the one who engaged the discussion on this topic in his pull request.

[cmdrrobin]

I have tested it and it works perfectly.Although I would suggest to add some comments why this is added.

[ccjmne]

Excellent suggestion! Done.