Update README.md

[lightmaster]

1. Added instructions to verify linux headers are installed, and to install the headers for your current kernel if they aren't already.
2. Added instruction that Proxmox uses a different name for its headers installation file.
3. Changed .deb installation instructions to apt install -f ./package.deb so apt installs .deb package and its dependencies in a single step.
4. Reformatted to improve the legibility of the various installation types and add links to the installation sections.