maintaining: add sub "Deprecation" section

MattSturgeon · MattSturgeon · commit 1350e87fa4d8 · 2025-05-23T22:02:06.000+01:00
diff --git a/MAINTAINING.md b/MAINTAINING.md
@@ -2,3 +2,6 @@
 
 This file is intended as a reference for Nixvim's core maintainers, although it may be interesting for anyone curious how we do certain things.
 
+## Deprecation
+
+The deprecation policy discussion is happening in [#3181](https://github.com/nix-community/nixvim/issues/3181).
