WilliamStar

I kept all the links in the original installation guide and followed the general structure of `zsh-autosuggestions` [Install.md](https://github.com/zsh-users/zsh-autosuggestions/blob/master/INSTALL.md). This PR would make the installation guides more uniform across different zsh...

I wanted to update the Install.md because I worked with both `zsh-autosuggestions` and `zsh-syntax-highlighting`, and I think their installation guide is more organized and easier to follow. Thus, I thought...

Changed "Mac OS" to "macOS"; All packages are now in alphabetical order; Manual Installation is at the top of the guide since it is preferred; `Oh My ZSH` is still...

ok... I removed the special case for Oh My Zsh. I think that my proposed organization and structure are better than the current one (that's why I made a pull...