DOC : edits to optional dependencies

 - cleaned up failed merge
 - added external movie programs to docs (closes matplotlib#2490)
 - added all the external libraries that are include in the source
   tree
 - added six as explicit dependency.  This was discussed before and
   rejected, but I added it here because we now have a minimum version
   check which should be documented.

Author: tacaswell

INSTALL

@@ -194,23 +194,9 @@ manager on Linux, you may need to install the development packages
     using pip, easy_install or installing from source, the installer
     will attempt to download and install `pyparsing` from PyPI.
 
-**Optional dependencies**
-
-`Pillow http://python-imaging.github.io/`__
-    If Pillow is installed, matplotlib can read and write a larger
-    selection of image file formats.
-
-
-libpng 1.2 (or later)
-    library for loading and saving :term:`PNG` files (`download
-    <http://www.libpng.org/pub/png/libpng.html>`__). libpng requires
-    zlib. If you are a Windows user, you can ignore this because we
-    build support into the matplotlib single-click installer
-
-:term:`freetype` 1.4 (or later)
-    library for reading true type font files. If you are a windows
-    user, you can ignore this since we build support into the
-    matplotlib single click installer.
+six 1.3 or later
+    Python 2/3 compatibility library.  This is also a dependency of
+    :term:`dateutil`.
 
 
 **Optional GUI frameworks**
@@ -234,6 +220,34 @@ backends and the capabilities they provide.
     The python wrappers for the wx widgets library for use with the
     WX or WXAgg backend
 
+**Optional external programs**
+
+ffmpeg/avconv or mencoder
+    Required for the animation module to be save out put to movie
+    formats.
+
+ImageMagick
+    Required for the animation module to be able to save to animated gif.
+
+**Optional dependencies**
+
+`Pillow http://python-imaging.github.io/`__
+    If Pillow is installed, matplotlib can read and write a larger
+    selection of image file formats.
+
+
+:term:`freetype` 1.4 (or later)
+    library for reading true type font files. If you are a windows
+    user, you can ignore this since we build support into the
+    matplotlib single click installer.
+
+
+libpng 1.2 (or later)
+    library for loading and saving :term:`PNG` files (`download
+    <http://www.libpng.org/pub/png/libpng.html>`__). libpng requires
+    zlib. If you are a Windows user, you can ignore this because we
+    build support into the matplotlib single-click installer
+
 **Required libraries that ship with matplotlib**
 
 :term:`agg` 2.4
@@ -244,6 +258,13 @@ backends and the capabilities they provide.
 `PyCXX` 6.2.4
     A library for writing Python extensions in C++.
 
+`qhull` 2012.1
+   A library for computing convex hulls.  Used for computing triangulation
+   and meshes.
+
+`ttconv`
+   truetype font utility
+
 .. _build_osx:
 
 Building on OSX