Theme doc needs an update

[Jmuccigr]

Based on recent changes to github-pages, it seems that the Themes doc needs an update. In particular

Your theme’s styles can be included in the user’s stylesheet using the @import directive.

@import "{{ site.theme }}";

should probably something more like:

Your theme’s styles can be included in the user’s stylesheet using the @import directive.

@import "jekyll-theme-<theme-name>";

This should match the theme indicated in the _config.yml file. I'm not quite sure I have this right, so I'm starting this discussion rather than just doing a PR.

See discussion here: pages-themes/cayman#131

All a result of github/pages-gem#772