Add project type to docs, fixes #1142

doc/04-schema.md

@@ -87,6 +87,12 @@ installer capable of installing packages of that type.
 Out of the box, composer supports three types:
 
 - **library:** This is the default. It will simply copy the files to `vendor`.
+- **project:** This denote a project rather than a library. For example
+  application shells like the [Symfony standard edition](https://github.com/symfony/symfony-standard),
+  CMSs like the [SilverStripe installer](https://github.com/silverstripe/silverstripe-installer)
+  or full fledged applications distributed as packages. This can for example
+  be used by IDEs to provide listings of projects to initialize when creating
+  a new workspace.
 - **metapackage:** An empty package that contains requirements and will trigger
   their installation, but contains no files and will not write anything to the
   filesystem. As such, it does not require a dist or source key to be