docs: Add link to Slackware packages

docs/global.rst.inc

@@ -21,4 +21,5 @@
 .. _Debian: http://packages.debian.org/attic
 .. _Ubuntu: http://packages.ubuntu.com/attic
 .. _Arch Linux: https://aur.archlinux.org/packages/attic/
+.. _Slackware: http://slackbuilds.org/result/?search=Attic
 .. _Cython: http://cython.org/

docs/index.rst

@@ -24,7 +24,7 @@ You can use pip to install |project_name| quickly and easily::
 
     $ pip install attic
 
-|project_name| is also part of the Debian_, Ubuntu_ and `Arch Linux`_
+|project_name| is also part of the Debian_, Ubuntu_, `Arch Linux`_ and Slackware_
 distributions of GNU/Linux.
 
 Need more help with installing? See :ref:`installation`.

docs/installation.rst

@@ -48,5 +48,5 @@ are normally bundled with the release tarball.
 Packages
 --------
 
-|project_name| is also part of the Debian_, Ubuntu_ and `Arch Linux`_
+|project_name| is also part of the Debian_, Ubuntu_, `Arch Linux`_ and Slackware_
 distributions of GNU/Linux.