Document that `fasd` is now bundled with prezto

Document that `fasd` is now bundled with prezto (19990c8025) and that it's a fallback to a manually installed version (04bfb5131b).

modules/fasd/README.md

@@ -15,8 +15,8 @@ been disabled.
 Installation
 ------------
 
-Since fasd is not an external module it needs to be installed.
+`fasd` is bundled with prezto as a git submodule. Alternatively, you can manually install `fasd`.
-`brew install fasd`
+If a manual install is found, it will be used instead of the bundled version.
 
 Aliases
 -------