diff --git a/doc/runtimes/nodejs.md b/doc/runtimes/nodejs.md
index 619e511845..5413c409f5 100644
--- a/doc/runtimes/nodejs.md
+++ b/doc/runtimes/nodejs.md
@@ -55,6 +55,16 @@ If a `yarn.lock` file is found in your repo root or if you specify "yarn" in
 the `engines` field of package.json, the latest or specified version of yarn
 will be used instead of npm.
 
+Note that **installing packages globally is unsupported**, whether requested directly
+by your app or by some pre/post install script of an included package. For example,
+this will **not** work in your `package.json`:
+
+```json
+  "scripts" : {
+    "preinstall" : "npm install -g somepackage"
+  }
+```
+
 # Run
 
 The following process is applied to determine how to start an app.