Add support for folder landing pages#336

Would be nice to be able to create landing pages for folders. Maybe this feature is already there, but I could not find it.

+1, a workaround is to create a doc called _README_, the underscores help it always be the first doc in the list but embedding the content in the folder page above the contents list would be excellent. Ditto for top-level wiki landing page

In https://github.com/pingcap/gdocwiki a README file will be automatically displayed for a directory