document rake watch in the README

doc-src/README.markdown

@@ -51,6 +51,13 @@ To compile (and auto recompile) and preview the site in your browser:
 
 Then open `http://localhost:3000/` in your web browser.
 
+If you find `bin/nanoc3 aco` to be sluggish, try this alternative workflow:
+
+    $ cd doc-src
+    $ export RUBYLIB="../lib:../../haml/lib"
+    $ serve 3000 .. &
+    $ rake watch
+
 ## Documentation on Nanoc
 
 * [Nanoc Homepage](http://nanoc.stoneship.org/)