AWS: Add documentation for x-amz-content-sha256 header

[msmoiz]

Thanks for creating this! It would be great to see documentation for the x-amz-content-sha256 header in the AWS signer docs. It seems this field needs to be set manually for an empty body (and perhaps in other cases) and this is not evident from the main example in the README or elsewhere. My original assumption was that this header would be set by the signer, and it took some time to figure out that I needed to set it myself.

[Xuanwo]

Thanks for raising this issue.

We require users to calculate and set the x-amz-content-sha256 to prevent reading the entire body for each request signature. This should be clearly stated in the documentation.

Would you like to initiate a PR for this?