Extend installation instructions in README.md?

[dirkbehme]

For the installation instructions in README.md I additionally had to

export LD_LIBRARY_PATH=$(rustup run 1.81.0 bash -c "echo \$LD_LIBRARY_PATH")

as explained in Experimental PR for introducing klint #958.

And from that link I found

make RUSTC=<path to klint binary>

quite helpful as well. Maybe both should be added in README.md?

And, btw, for the given

rustup run 1.81 klint

I get

$ rustup run 1.81 klint
error: toolchain '1.81-x86_64-unknown-linux-gnu' is not installed

while I built klint as given in the instructions before this successfully. Confusing ...