let the rails gem ship with the README

The doc:rails application rake task needs this README.

rails.gemspec

@@ -18,7 +18,7 @@
 
   s.bindir      = 'bin'
   s.executables = []
-  s.files       = Dir['guides/**/*']
+  s.files       = ['README.rdoc'] + Dir['guides/**/*']
 
   s.add_dependency 'activesupport', version
   s.add_dependency 'actionpack',    version