Added missing example to controllers.md
Typo fix in upgrade.md
Document registering command from a service provider.
Merge pull request #775 from fareez-ahamed/4.1

Typo fix in upgrade.md
Merge pull request #743 from YavorK/patch-2

Added missing example to controllers.md
Add note about serializing objects.
Merge branch '4.1' into 4.2