Docs for multipart request with in memory file contents

[pdgonzalez872]

Hi @edgurgel! Thanks for the library, it is my go to when dealing with requests.

I had a task a while back that dealt with fetching a file from S3 and uploading it elsewhere. The docs we currently have only show the workflow where you have the file locally, not in memory. The linked issues in that section don't mention this use case, so I think it would be helpful to add it to the readme. I'll create a PR soon, for it, if you think it adds value, feel free to merge it. Those docs would have saved me a bunch of time, maybe other folks can benefit from what we already learned.

cc: @costaraphael, that issue you helped me with a while back!

Thanks again for the lib!