Update README.md to explain custom file use case

Adding documentation to indicate a use case for custom file values and options in multipart form data requests. The added sentence may have saved my team a few hours.

Use case:
- A user wants to upload a file to your service, and you in turn want to upload the file to an external service.
- Constraint: you don't want the file to be written to disk in your service.
- Because you're uploading a file from a stream, you need to add custom file-related information manually in your request to the external service.
Merge pull request #1288 from cliffcrosland/patch-1

Update README.md to explain custom file use case