1. Replaced contributing.html page with contrib/ directory containing separate pages for different ways to contribute.
2. Fixed references to contributing.html.
3. Added main menu pulldown for different ways to contribute.
1. Rewriting guide to creating exercises and examples based on feedback from @pipitone.
2. Updating description in home page of what we need.
3. Adding definition to standard configuration YAML for swcarpentry.github.io (rendered pages).
Merge branch 'getting-involved'
Small changes to wording on index page for contributions