Add a friendly top comment about nix-installer.sh

[Hoverbear]

Description

Some folks were a bit curious what the nix-installer.sh did, so we reworded the start to make it more friendly and explain quite directly what it does, as well as offering users a way to avoid using the script entirely.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install