[Request] Add filename extension requirement to mpu.io.write() and read()

[OutsourcedGuru]

I had to deep-dive the source code to see why my mpu.io.write() was silently just writing a zero-byte size output to a file with a filename format like: 20200124022008221627_2. Since my output file didn't have a file extension, your code fell to this line.

Could you update the README.md at the end to have a little more information about your expected file extensions? Thanks.