minor README re-org
minor README re-org

Move 'bootstrapping' section up to top (since it is fundamental and
short) and 'service interfaces' down a ways (below 'built-in services')
since it is a bit more about things that are *possible* vs. things that
are *necessary*.

rbramwell/trapperkeeper,scotje/trapperkeeper,senior/trapperkeeper,puppetlabs/trapperkeeper,senior/trapperkeeper,camlow325/trapperkeeper