Add Nix installation method

app/views/docs/_script_install_and_other.html.erb

@@ -1,2 +1,2 @@
 <%= render partial: 'docs/script_install' %>
-<p>Other installation options, including Homebrew and distro packages (Ubuntu, Fedora, Arch Linux, Gentoo), are <a href='<%= docs_path(:installation) %>'>also available</a>.</p>
+<p>Other installation options, (Homebrew, Ubuntu, Fedora, Arch Linux, Nix, Gentoo), are <a href='<%= docs_path(:installation) %>'>also available</a>.</p>

app/views/docs/installation.html.md

@@ -54,6 +54,12 @@ Gentoo Linux users can install asciinema from
     layman -a go-overlay
     emerge -av dev-go/asciinema
 
+### Nix / NixOS
+
+If you're using Nix package manager then we have you covered too:
+
+    nix-env -i asciinema
+
 ### Ubuntu
 
 To install Ubuntu package add