documentation: large temporary file usage

[pts]

Add something like this to README.md:

Limitation: Since adbfs-rootless uses adb push and adb pull under the hood to copy files, a temporary local file will be created on the computer for each open file. This is impractical for very large files (say, more than 1 GiB) because of the large temporary storage requirement on the computer and the high open latency.