Added note about dependencies.

README.textile

@@ -79,6 +79,10 @@ The best way to install Jekyll is via RubyGems:
 
   $ sudo gem install mojombo-jekyll -s http://gems.github.com/
 
+Jekyll requires the gems `directory_watcher`, `liquid`, `open4`, 
+and optionally `rdiscount` for markdown support. These are automatically
+installed by the gem install.
+
 h2. Run
 
   $ cd /path/to/proto/site