Add Gentoo installation instructions to docs

app/views/docs/getting-started.html.slim

@@ -12,7 +12,7 @@ markdown:
       sudo pip install --upgrade asciinema
 
   See [installation docs](#{docs_path(:installation)}) for more installation
-  methods (Ubuntu, Arch Linux).
+  methods (Ubuntu, Arch Linux, Gentoo).
 
   ## 2. Record
 

app/views/docs/installation.html.slim

@@ -29,10 +29,17 @@ markdown:
   ## Arch Linux
 
   Arch Linux users can install the
-  [AUR](https://aur.archlinux.org/packages/asciinema/) package:
+  [AUR package](https://aur.archlinux.org/packages/asciinema/):
 
       yaourt -S asciinema
 
+  ## Gentoo Linux
+
+  Gentoo Linux users can install the
+  [portage package](http://packages.gentoo.org/package/app-misc/asciinema):
+
+      emerge asciinema
+
   ## No native package for your operating system?
 
   If you use some other operating system and you know how to build a native