Document the default storage backend to the README

[betelgeuse]

cache-handler/README.md

Lines 6 to 9 in 3c8632e

	> [!WARNING]
	> Since `v1.7.0` Souin (the development repository that cache-handler is based on) implements only one storage. If you need a specific storage you have to take it from [the storages repository](https://github.com/darkweak/storages) and add it either in your code, during the build otherwise.
	(e.g. with otter using caddy) You have to build your caddy module with the desired storage `xcaddy build --with github.com/caddyserver/cache-handler --with github.com/darkweak/storages/otter/caddy` and configure otter in your Caddyfile/JSON configuration file.
	See the [documentation about the storages](https://docs.souin.io/docs/storages).

The link does not reveal which one the one storage is. The README would be improved by documenting this directly.

[nickchomey]

Seems to be an in-memory sync.Map