Bring back notice about maintenance only

Seems this was removed by accident in e4d606f#diff-7b3ed02bc73dc06b7db906cf97aa91dec2b2eb21f2d92bc5caa761df5bbc168f.

Author: nicoddemus

README.rst

@@ -4,6 +4,9 @@ execnet: distributed Python deployment and communication
 Important
 ---------
 
+**execnet currently is in maintenance-only mode, mostly because it is still the backend
+of the pytest-xdist plugin. Do not use in new projects.**
+
 .. image:: https://img.shields.io/pypi/v/execnet.svg
     :target: https://pypi.org/project/execnet/