Further tidy-up based on email feedback.

1. Moved the software installation instructions down so that "how I set up" is the first thing readers see.
2. Changed titles.
3. Changes "customization instructions below" to link to `CUSTOMIZATION.md`.
4. Changed nested of software installation instructions so that PyYAML wasn't under Jekyll.
Merge pull request #42 from gvwilson/tidying-up-wording

Further tidy-up based on email feedback.