add instruction to build the binary on debian to README.md

[freddii]

install cargo:
sudo apt install cargo

a) create the binary:
git clone https://github.com/Soft/xcolor
cd xcolor
make

after that the build binary is located in ./target/release/xcolor

b) or create a deb file:
cargo install cargo-deb
git clone https://github.com/Soft/xcolor
cd xcolor
cargo deb
after that the build deb file is located in ./target/debian/xcolor_0.5.0_amd64.deb

[freddii]

sadly my instruction does not create a really usefull deb file.
So there might be maybe a better solution to build a working deb file.