Update installation instruction and include more packages [skip ci]

docs/installation.rst

@@ -6,12 +6,17 @@ Install the extension with one of the following commands::
 
     $ easy_install sigal
 
-or alternatively if you have pip installed::
+or preferably if you have pip installed::
 
     $ pip install sigal
 
 To install the development version, see the :doc:`contribute`.
 
+Also, note that Sigal depends on `Pillow
+<https://github.com/python-pillow/Pillow>`_, which needs specific libraries when
+building from sources. See their `Installation documentation
+<https://pillow.readthedocs.io/en/stable/installation.html>`_ for more details.
+
 Dependencies
 ~~~~~~~~~~~~
 
@@ -25,4 +30,10 @@ Dependencies
 Packages
 ~~~~~~~~
 
-- Archlinux: `AUR package <https://aur.archlinux.org/packages/sigal/>`_
+Some packages are available for Linux distributions:
+
+- `Archlinux <https://aur.archlinux.org/packages/sigal/>`_
+- `Gentoo <https://packages.gentoo.org/packages/media-gfx/sigal>`_
+- `OpenSuse
+  <https://build.opensuse.org/package/show/openSUSE:Factory/python-sigal>`_
+- `NixOS <https://nixos.org/nixos/packages.html#sigal>`_