Client-browser readme is missing fallback strategies and alternatives

The fallback section of the readme for the client-browser module lists the features of the SDK that are not supported by certain browsers. However, while it mentions `msrcrypto` as an example of a fallback library, it doesn't recommend any fallback strategies or alternatives.

Also, the fallback section points to @aws-crypto/web-crypto-backend, but that file has very little information, and just sends readers back to the readme. Loops like this are very frustrating to users.

Inizia dal README del modulo client-browser, in particolare dalla relativa sezione sul fallback, e analizza il file @aws-crypto/web-crypto-backend a cui fa riferimento. Documenta strategie pratiche di fallback e alternative, e assicurati che il file referenziato fornisca informazioni utili senza rimandare i lettori a un loop.

Scritto dal modello di indicizzazione a partire dal testo della issue.