README.md should explain that `g:nv_main_directory` must also appear in `g:nv_search_paths`

[horasio]

All is in the title: README.md should explain that g:nv_main_directory must also appear in g:nv_search_paths

(Or this should be included automatically if not there, or something.)

I struggled for a while when none of my documents in the g:nv_main_directory were searched in... until I added that directory in the g:nv_search_paths variable.
Looks like a great tool, thanks !