Detailed usage in README

I was going to create a custom signature for global id, but then I found that there is a `locate_signed` method in the sources.

I think it would be good to describe these features in README.
This could still be clearer :smile:
Just showing the behavior is not enough. You need to explain that a `GlobalID::Locator.locate` finds an app based on the app in the `gid://` url. So a `use` call binds a locator to a specific app that the locator can find.
(I would put this explanation where you're first describing the feature.)