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.

Start with the client-browser module's README, especially its fallback section, and inspect the @aws-crypto/web-crypto-backend file it references. Document practical fallback strategies and alternatives, and ensure the referenced file provides useful information without sending readers back in a loop.

Written by the indexing model from the issue text.