Create 20-troubleshooting.html.md

Problems I found when porting from Flixel to HaxeFlixel that had me puzzled for a while.
Update 20-troubleshooting.html.md
Update 20-troubleshooting.html.md

Some further writing
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Final draft
Update 20-troubleshooting.html.md

Clarified compiler flags, added something about timestep
Update 20-troubleshooting.html.md

Moved timestep to the top, changed wording a bit.
Merge pull request #71 from BNeutral/patch-1

Troubleshooting page