Merge pull request JohnSundell#40 from JohnSundell/plugin-return-exis…

…ting-instance

Plugins: Return existing instance when adding a plugin

Actor: Document that scale now affects collision detection

Merge pull request JohnSundell#30 from JohnSundell/tutorial-2

Documentation: Add 2nd tutorial

README: Update for macOS support

Tutorial JohnSundell#1: Add missing swift syntax annotation to code b…

…lock

README: Add note on platform support