DOC: Add note that NEP 29 is superseded by SPEC 0

The SPEC is basically the predecessor, we should try to slowly
refer to it mostly, so add a note here.

Author: seberg

doc/neps/nep-0029-deprecation_policy.rst

@@ -11,6 +11,9 @@ NEP 29 — Recommend Python and NumPy version support as a community policy stan
 :Created: 2019-07-13
 :Resolution: https://mail.python.org/pipermail/numpy-discussion/2019-October/080128.html
 
+.. note::
+  This NEP is superseded by the scientific python ecosystem coordination guideline
+  `SPEC 0 — Minimum Supported Versions <https://scientific-python.org/specs/spec-0000/>`__.
 
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