README: add more information, and link to slides

[waldyrious]

Slideshow presentation: http://jeromeetienne.github.io/slides/doccoviewer.parisjs20/

[waldyrious]

In fact, since the master branch is now rendered by github pages, perhaps renaming index.html would leave the README as the github-pages landing page, from which we could link to the demo. That would make both views (github pages and source) equally informative, and automatically in sync, which neatly aligns with the project's philosophy.