AWS: Add documentation for x-amz-content-sha256 header #447

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.