DOC: add note about IPython support window

Closes matplotlib#16263

Author: tacaswell

doc/devel/min_dep_policy.rst

@@ -51,6 +51,13 @@ versions no longer support our minimum NumPy or Python.
 
 We will work around bugs in our dependencies when practical.
 
+IPython and Matplotlib do not technically depend on each other, however there
+is practical coupling between the projects.  There is no dependency to update,
+but we will ensure the integration works with at least minor or major versions
+of IPython released in the 24 months prior to our planned release date.  We
+will not warn if used within an IPython outside of this window.
+
+
 
 Test and documentation dependencies
 ===================================