Type fix
Updared extensions guide to include info about PSR-4 autoloading
Merge pull request #1877 from plusodinminus/patch-2

Type fix
Updated extension guide to suggest stricter naming of packages
Made extensions guide more human-friendly and more specific
And a bit more human-friendly extensions doc
Typo in MessageController [skip ci]
Merge pull request #1879 from lucianobaraglia/master

Typo in MessageController [skip ci]
Added spanish translation
Merge pull request #1880 from lucianobaraglia/spanish-message-translation

Added spanish translation
Second commit for #1874