diff --git a/README.md b/README.md
index d99ea22..dd26353 100644
--- a/README.md
+++ b/README.md
@@ -48,6 +48,9 @@ These are hard constraints that shape every decision:
 - Kate 25.12.3, KDE Frameworks 6, Qt 6
 - C++20, CMake, extra-cmake-modules
 - C++ is the only first-class plugin path on this install (no Python/Pâté binding present)
+- [plan9port](https://9fans.github.io/plan9port/) — the `plumb` plugin is a
+  plumb client and talks to plan9port's plumber at runtime (see
+  [docs/PLUMBING.md](docs/PLUMBING.md)). Only needed if you enable `plumb`.
 
 ## Build