Add some documentation of structure to README

[dcwalk]

I think we need some documentation about structure. Here are some things I think might need to be addressed:

• where do (different) files go (img, docs like reports or presentations...)?
• how to make a change to navigation (either header/footer)
• some blurb about _assets and _includes

[dcwalk]

Addressed in https://github.com/ixmaps/website2017/commit/0554e82c4b1f154ce25cdea26b2c4505d414c3f6 and refined. Good for now, but sorely needs expanding.