Create user-documentation folder and 2xfor1x stub

[manez]

For #71

[ruebot]

Thanks @manez! A one more item we need to update...

Can you update mkdocs.yml, and add a section for User Documentation?

It'd look something like this:

# User Documentation
  - ['user-documentation/2xfor1x.md', 'User Documentation', 'Islandora 2.x for Islandora 1.x Users']

Then, @dmoses can add to mkdocs.yml and this directory as he goes along.

[manez]

Done and done. I hope.

[ruebot]

Perfecto!