Define the range of supported kernel versions in the README

[drygdryg]

Problem: now I see that kernel versions up to 5.8 are supported, and I want to build driver version 5.3.9 with the kernel 4.9 (4.9.238), but I get errors (you can see a part of build log here: https://pastebin.com/YSeiw2GJ) Solution: define the range of supported kernel versions in the README, e.g. "kernel 4.19 to 5.8 supported". I think this will help reduce the number of building-related issues asked here.

BTW, is it possible to build this driver with the kernel 4.9?