Merge pull request #896 from michaelmcandrew/patch-1

"Out of the box, composer supports three types" (not two types)...

doc/04-schema.md

@@ -78,7 +78,7 @@ that needs some special logic, you can define a custom type. This could be a
 all be specific to certain projects, and they will need to provide an
 installer capable of installing packages of that type.
 
-Out of the box, composer supports two types:
+Out of the box, composer supports three types:
 
 - **library:** This is the default. It will simply copy the files to `vendor`.
 - **metapackage:** An empty package that contains requirements and will trigger