Clarify that root package depends on context

doc/04-schema.md

@@ -23,6 +23,11 @@ The config of dependencies is ignored. This makes the `config` field
 If you clone one of those dependencies to work on it, then that package is the
 root package. The `composer.json` is identical, but the context is different.
 
+> **Note:** A package can be the root package or not, depending on the context.
+> For example, if your project depends on the `monolog` library, your project
+> is the root package. However, if you clone `monolog` from GitHub in order to
+> fix a bug in it, then `monolog` is the root package.
+
 ## Properties
 
 ### name