Update installation guide to cargo install.

[KighteW3]

Hi, when i tried to install leftwm-theme by the git repo i had many issues compiling with

cargo build --release

(i have already installed all the stuff like kernel-devel, openssl, openssl-devel, rustup, gcc, makefile, etc...).

Then i used

cargo install leftwm-theme

and it worked without any problem or warning, so i suggest to update the README.md to add this way of installation.

[mautamu]

Hey @KighteW3!

cargo install leftwm-theme instructs Cargo to build what's in crates.io; installing using cargo install --path . from within this directory tells Cargo to install what's here. Wondering if there's something going on with the vendoring that would cause it to fail to build. Are you able to build the latest master now?