Merge pull request #78 from pvdb/fix_contributing_documentation

Fix contributing documentation: install "hoe" gem dependency

Author: hsbt

CONTRIBUTING.rdoc

@@ -10,10 +10,14 @@ http://github.com/ruby/rake . The public git clone URL is
 If you wish to run the unit and functional tests that come with Rake:
 
 * +cd+ into the top project directory of rake.
+* Install the +hoe+ gem dependency:
+
+    gem install hoe # Unless the hoe gem is already installed
+
 * Type one of the following:
 
-    rake newb    # If you have never run rake's tests
-    rake         # If you have run rake's tests
+    rake newb       # If you have never run rake's tests
+    rake            # If you have run rake's tests
 
 = Issues and Bug Reports