Adding notes about using imports in application manifest and file extensions

Great work on the guide so far, lots of useful info.

Just adding a few notes here about how to use `@import` in a layout manifest, as it can't be used with the default generated one. There's one other thing I'd like to add, which is details of whether an `application.css.scss` manifest needs to use `@import "users";` to explicitly include other stylesheets (or some equivalent for a directory) or if other sass stylesheets are implicitly imported (I can't think why they would be). Can someone who knows review this? /cc @ryanfrenchNZ