Fixes 498 - adds graphic source directory

Adds root directory for graphic assets source files and includes a
readme for the directory.
This is not entirely accurate. To view the layout that the tests see, you would also need to change `config.default_per_page` in `config/kaminari_config.rb` to `1` for the pagination to work properly. So, I would remove the notes about tests, and just say "if you want to view a different number of results per page, just add `&per_page=1` or similar to the end of the URL."