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.
Sorry, I made a mistake in my last comment :smile:
`locate` doesn't find an *app*, it finds a *locator* for an app based on the app in the `gid://` url.

Revise the text to:
### Custom App Locator
A custom locator can be set for an app by calling `use` with the app and the locator to use. A custom app locator is useful when different apps collaborate and reference each others' Global IDs.
The locator to use, when a Global ID's model is being found, is based on the app in the Global ID url.

A custom locator can either be a block or a class.

...etc.