[1.x] docs: move hand-written content to README.md

This is my attempt to improve the documentation, based on our discussions in the past.

 - The README.md published to npmjs.org contains all information needed to use the module.

 - The apidocs rendered at http://apidocs.strongloop.com/loopback-boot/ includes the README, so that the doc page contains all relevant information in one place. As a side effect, the caption includes package version now (the version is appended by strong-docs to the h1 element(s)).

 - I have extended the documentation in README to describe all configuration files and conventions.

Note: the content describes the old 1.x structure of the project. I'll submit another PR for the 2.0 branch after this one is approved.

/to @ritch @crandmck please review.