correct path for 'working with the source' page

It looks like the 'Working with the Source' page was renamed when migrating to GitHub. This change correct the link (like in 'Contributing' page), but maybe its name should be corrected too.
SelectBoxStyle.background is optional. In that case the SelectBox
background is transparent and user sees its parent actor background.
When calling IntFloatMap or IntIntMap getAndIncrement(..) with '0' key
the returned value must be the value before increment, not the value
after.
Merge pull request #2258 from guillaume-alvarez/intmaps

correct return value for IntFloatMap and IntIntMap getAndIncrement with 0 key
Fixed #2212 by adding a flag indicating whether to throw exception
Update CHANGES
Merge pull request #2255 from guillaume-alvarez/patch-2

SelectBoxStyle.background is optional.
Fix shader compilation crashing on Android
Merge pull request #2254 from guillaume-alvarez/patch-1

correct path for 'working with the source' page
Update Ashley version

Closes #2267
Dispose potPixmap

Closes #2266
Closes  #2246
Fixed OpenALMusic leak. Fixed stutter from OpenALMusic#play() while playing.

closes #2233
Merge remote-tracking branch 'origin/master'
Cancel touch focus for DragAndDrop.

closes #2240
Call stop before the onCompletion callback

Closes #2268
Added randomSign.

closes #2272
Merge remote-tracking branch 'origin/master'
Rename move GamepadTest to ControllersTest

Also moved to extensions package, since it is an extension.

Closes #2231
FSM changeState possible with null.