Add basic documentation on external module directories

README.md

@@ -90,6 +90,19 @@ accompanying README files to learn of what is available.
 
      ![sorin theme][2]
 
+### External Modules
+
+  1. By default modules will be loaded from */modules* and */contrib*.
+  2. Additional module directories can be added to the
+     `:prezto:load:pmodule-dirs` setting in *~/.zpreztorc*.
+
+    Note that module names need to be unique or they will cause an error when
+    loading.
+
+     ```console
+     zstyle ':prezto:load' pmodule-dirs $HOME/.zprezto-contrib
+     ```
+
 Customization
 -------------