Mark ColormapRegistry as experimental.

Author: timhoffm

doc/users/next_whats_new/colormaps.rst

@@ -1,8 +1,9 @@
-Colormap registry
-------------------
+Colormap registry (experimental)
+--------------------------------
 
 Colormaps are now managed via `matplotlib.colormaps`, which is a
-`.ColormapRegistry`.
+`.ColormapRegistry`. This API is experimental and may still change in future
+versions.
 
 Colormaps can be obtained using item access::
 

lib/matplotlib/cm.py

@@ -102,6 +102,10 @@ class ColormapRegistry(Mapping):
     r"""
     Container for colormaps that are known to Matplotlib by name.
 
+    .. admonition:: Experimental
+
+       This API is experimental and may still change in future versions.
+
     The universal registry instance is `matplotlib.colormaps`. There should be
     no need for users to instantiate `.ColormapRegistry` themselves.