uinput: note that UI_SET_EVBIT configuration is always needed

If you don't add a UI_SET_EVBIT configuration entry to a register
command, the other configuration entries will just be silently ignored.
This isn't very obvious to someone not familiar with the uinput Kernel
API, so let's add a note about it in the README.

Bug: 245989146
Test: check rendering in Gitiles
Flag: DOCS_ONLY
Change-Id: I290bafb276a8caf0639f062eff16aeff32bac8d0