Avoid installing uv by blindly executing a script

[Martin1887]

Blindly executing a script is a potential security issue easy to avoid. Describing its potential harness may not only avoid installing malware in case the official URL is compromised but also teach a basic security measure to the readers.

I am not sure where this README.md is shown, but I think that something like this should be used for all installation instructions.

Note: Other methods to install uv exist, like Nix, devenv.sh or compiling with cargo, but listing all installation methods is probably overwhelming and overkill.