Add Linux installation instructions

This adds installation instructions for installing atom on Linux. It advises the use of [Nix](https://nixos.org/nix), which has the advantages that it doesn't require root and works on every Linux distribution. (The [Nix atom pkg](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/atom/default.nix) should be updated sometime though)

It also mentions the .deb file on the release page.

What do you think?