Can we? Yes
Should we? Sometimes yes.

I do it for many of my uploads. Most times it contains the same information as the online details on GrabCad. Sometimes it has a little more information like print settings if it’s for 3D printing or a little history of the design. Sometimes a README.TXT is just an index for all the other included documentation.

If the upload consists of many interrelated and/or complicated components then I would consider a README.TXT mandatory. If the upload is a very simple self explanatory part then the inclusion is probably unnecessary.