Revamp README, safer gh-pages branch creation steps

[ches]

• Safer approach to creating gh-pages branch that avoids questions of when to switch back to the real development branch in your original working clone, etc.
• Try to make the distinction between the roles of sbt-site and sbt-ghpages more clear, so the order of setup is more obvious.
• Assume less knowledge of how GitHub Pages works, to uncover a little of the magic of what the plugin is doing. This should also help assure users that dirty files in their working clone won't get bungled.

Closes #9, #15

[lightbend-cla-validator]

Hi @ches,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Typesafe Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[ches]

(Completed the CLA approval).

[eed3si9n]

Merged