Update cheat-sheet.md

Adds more todo's
Update cheat-sheet.md

Fills in "Why testing", "TDD" and some "General advice".
Update cheat-sheet.md

minor layout changes
Merge branch 'patch-1' of git://github.com/BernhardKonrad/boot-camps into BernhardKonrad-patch-1

Conflicts:
	testing/cheat-sheet.md