Add Content-Range Header Requirement to Docs

[steverecio]

I was setting up some custom client-side code to interact with the chunked upload API and during the debugging process I discovered the requirement for setting the Content-Range header in the chunk uploading process only after digging through the source code. Would be great to add this more explicitly in the docs in Step 3 of Typical Usage

[jerinpetergeorge]

@steverecio It's a good suggestion. I am also planning to add a minimal python example regarding how to use this upload API.