Add a tips section

Even though running Fleet is pretty straight forward, if you have multiple instances running on the same server you will need to be aware that it consumes multiple ports.
Clarified what each port is used for.
Merge pull request #47 from ccowan/master

Added a "tips" section
added fleet ps --json example to the readme
added missing line break before code fence in fleet ps --json example
Merge branch 'master' of git://github.com/substack/fleet
Merge branch 'master' of git://github.com/nisaacson/fleet