add `autocmd ColorScheme` info

[mmlb]

@mhinz dropped this piece of info so casually but it is great and should be added to vim-galore.

Where to put though?

• ColorSchemes - makes sense because users can be expected to want to tweak a couple of things in colorschemes
• Autocmds - is pretty nice because it hints at another class of filetypes that vim recognizes (I always just thought it was by ft)
• Tips - because it is a minor bit of info.

[mhinz]

Hi, I wanted to keep it as short as possible, so I put it into the Tips section, but also added a link to it from the Basics section, where the usage of :hi is explained etc.

Thanks for bringing this up!