Add link to Development Environment Setup in the readme.

Point people who need to build this from source in the right direction.
Merge remote-tracking branch 'axujen/master'
Fix the frame update not getting called on the first frame
Merge remote-tracking branch 'scrawl/master'
Avoid trying to find a state for non-actors.

They're only ever play idles, unless PlayGroup is called.
Playing with moons part 2
Corrected celestical object sizes
Merge remote-tracking branch 'glorf/weather'
Remove a duplicate showWeapons call
Fix vec.z being applied while not in the air
Play the item up/down sound when readying/unreadying weapons
Don't clear the last queued animation when it stops
Merge remote-tracking branch 'zini/master' into animations