diff --git a/docs/getting-started/installation.md b/docs/getting-started/installation.md
index b61606ba..b0309b57 100644
--- a/docs/getting-started/installation.md
+++ b/docs/getting-started/installation.md
@@ -135,3 +135,12 @@ These directories need to be writable:
 - `storage/framework/session`
 
 - `storage/framework/views`
+
+If you use the addon/module system (Admin → Addons), these two directories
+also need to be writable by your webserver user. They aren't covered by the
+installer's requirements check, since addons are installed after initial
+setup:
+
+- `modules`
+
+- `public/ext`