Update rich_html.rb

Without assigning value to symbol :add_help it will not run under ruby 1.8.7.  Both 1.9.3 and 2.0.0 will accept the symbol without any assignment.
The `bundle exec rake wagn:migrate` command will fail (with syntax error, unexpected tASSOC, expecting ']') without this patch.
Merge pull request #98 from tukanos/master

Adding symbol :add_help into hash.  The migration at ruby 1.8.7 will fail