Update digFlood documentation to mention onLoad(World).init

Per http://www.bay12forums.com/smf/index.php?topic=139553.msg5712785#msg5712785
Merge pull request #355 from lethosor/patch-6

Update digFlood documentation to mention onLoad(World).init
Merge branch 'develop' of github.com:DFHack/dfhack into develop