Add note about requiring a JDK

OS X Mavericks has removed the automagic installer so this is a little less trivial now and probably worth a mention.
Add instructions for installing Maven

Yes Mavericks ditches this also...
Encourage user to check if they have mvn installed
Typo :rage3:
Specify Java 1.6 for JDK install
Correct typo :blush:
Merge pull request #5 from seadowg/jdk-note

Update docs for Mavericks