Adds documentation in readme.md for currently undocumented feature

When I ran up ConfigInjector in an existing MVC4 app, it threw exceptions like "ExtraneousSettingsException: Setting webpages:Version appears in [web|app].config but has no corresponding ConfigurationSetting type".

I don't care to create classes for those settings, so I wanted to ignore them.  I could not see the feature to ignore documented so I browsed the source and quickly found it was there.

This pull request is just to include a new FAQ to help others that experience the same issue as I did.  I hope the language is appropriate and consistent with the rest of the documentation.