diff --git a/src/extensions/README b/src/extensions/README
--- a/src/extensions/README
+++ b/src/extensions/README
@@ -1,2 +1,10 @@
 You can add `.php` files to this directory to load classes and functions defined
 in them at runtime.
+
+Any such files are loaded automatically and unconditionally, without the need
+to `arc liberate` first to update a library map. You can also extend Phorge by
+creating a separate library, storing it somewhere else, and adding it to the
+`load-libraries` key in configuration.
+
+See more documentation on how extensions work at
+https://we.phorge.it/w/docs/extensions/install/