Add FAQ section to README

[jemag]

Hi, just thought perhaps this could help newcomers who are having troubles setting up the plugin. Feel free to ignore if you don't feel like this is valuable.

[lukas-reineke]

Thank you. I think it is a good idea to add something like this.
I am just wondering if users don't use/know colorcolumn, it would probably be easier to just use require("virt-column").setup { virtcolumn = 120 }.
Then there is also no need to mess with any highlight groups.

Maybe the FAQs should explain both 🤔