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.

从 client-browser 模块的 README 开始，尤其是其中的 fallback 部分，并检查它所引用的 @aws-crypto/web-crypto-backend 文件。记录实用的 fallback 策略和替代方案，并确保被引用的文件提供有用信息，而不是让读者回到循环中。

由索引模型根据 Issue 内容生成。