Removing all instructor's guide material from the main repository
Removing note about online office hours from home page
CONTRIBUTING.html -> CONTRIBUTING.md

I was looking for some information about contribution and found the broken link. Please let me know (and I'm sorry) if .html is proper extension for the reason that I am unaware of.
Merge pull request #393 from gvwilson/removing-instructors-guide-from-main-repo

Removing all instructor's guide material from the main repository
Merge pull request #399 from gvwilson/removing-online-office-hours-from-home-page

Removing note about online office hours from home page
Adding links to lists to our 'contributing' page
Adding 'our audience' to main menu
Merge pull request #406 from gvwilson/adding-link-to-audience-page-to-menu

Adding 'our audience' to main menu
Fixing typo (double 'us')
Merge pull request #405 from gvwilson/adding-link-to-other-mailing-lists

Adding links to other mailing lists to our 'contributing' page