Add note about requiring composer for plugin dev, fixes #6132

doc/articles/custom-installers.md

@@ -85,10 +85,16 @@ Example:
     },
     "require": {
         "composer-plugin-api": "^1.0"
+    },
+    "require-dev": {
+        "composer/composer": "^1.3"
     }
 }
 ```
 
+The example above has Composer itself in its require-dev, which allows you to use
+the Composer classes in your test suite for example.
+
 ### The Plugin class
 
 The class defining the Composer plugin must implement the