Add a Notice Regarding Linux Kernel Modules in the README

[thekief]

Having success with some other C source code, I tried to port a Linux kernel module from C to Rust, not being aware about C2Rust's inability. I was able to get the compile_commands.json file and tried to transpile the code with c2rust. The execution didn't yield any output but I haven't seen any errors either.

Only after echoing the returns status, which was a 1, I looked into the issues and found the issue #370 . In the light of the Linux kernel gaining better Rust support release after release some more people might run into the same issue and I think it would be good to add a notice that only userland code is fully supported.