Gem installation instructions updated

Using "gem install" didn't work for me because of the Gemfile.lock that is in source control (presumably for a reason).

README.markdown

@@ -95,9 +95,9 @@ Assuming you're already within your project's clone directory, and you've alread
 
 ### Install jekyll if you have not already
 
-> **Note:** Jekyll 1.1.2 is a known good version.
+> **Note:** Jekyll 1.1.2 is a known good version, and it is specifically referred to in `Gemfile.lock` so you have to use `bundle` (not `gem install ...`) to install it:
 
-    gem install jekyll
+    bundle
 
 ### Run jekyll