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.
I'd revise the text to:
Finding models for either the `foo` or `bar` app will now use the custom locators.
```ruby
GlobalID::Locator.locate "gid://foo/Person/1" # => model found with the block locator
GlobalID::Locator.locate "gid://bar/Person/1" # => model found with `BarLocator`
GlobalID::Locator.locate "gid://some-app/Person/1" # => model found with default locator
```