add install command on README.md

[bigzaqui]

👋 thanks for building this tool!

I had to spend a couple of minutes learning how to install the tool since is not in crates.io and the branch is not master, to save the time to other people, would be nice to have it as part of the README.md

cargo install --git https://github.com/softprops/fasttime fasttime --branch main

thanks!