README.md: Update link to SWC contributing page
Catch up with the swcarpentry/website -> swcarpentry/site transition,
specifically
86ba191 (Replaced contributing.html page with contrib/
directory containing separate pages for different ways to contribute,
2013-08-22).
Instructor notes for teaching git via collaboration on GitHub.
The focus of the lesson is to have students contribute to a repo on GitHub in
which we are building up a list of the things they are most interested in
learning at the boot camp. Requires everyone have a good internet connection
and that at least some of the participants sign up for GitHub.