DOC: add minimum dependency version policy in writing

doc/devel/index.rst

@@ -21,6 +21,7 @@ The Matplotlib Developers' Guide
    gitwash/index.rst
    coding_guide.rst
    release_guide.rst
+   min_dep_policy.rst
    MEP/index
 
 .. toctree::

doc/devel/min_dep_policy.rst

@@ -0,0 +1,42 @@
+.. _min_deps_policy:
+
+======================================
+Minimum Version of Dependencies Policy
+======================================
+
+Python and numpy
+================
+
+- support minor versions of ``Python`` initially released in the previous
+  3 years
+- support minor versions of ``numpy`` initially released in the
+  previous 3 years or oldest that supports the minimum python version
+  (which ever is higher)
+
+We will bump the minimum python and numpy versions as we can every
+minor and major release, but never on a patch release.
+
+Python Dependencies
+===================
+
+For python dependencies we should support at least
+
+with compiled extensions
+  minor versions released in the last 3 years
+  or the oldest that support our minimum python + numpy
+
+without complied extensions
+  minor versions released in the last 2 years or the oldest that
+  supports our minimum python.
+
+We will only bump these dependencies as we need new features or the
+old versions no longer support our minimum numpy or python.
+
+
+System and C-dependencies
+=========================
+
+For system or c-dependencies (libpng, freetype, GUI frameworks, latex,
+gs, ffmpeg) support as old as practical.  These can be difficult to
+install for end-users and we want to be usable on as many systems as
+possible.  We will bump these on a case-by-case basis.