fix: document outDir Forge Config value (#214)

Author: eliotschu

config/configuration.md

@@ -63,7 +63,8 @@ module.exports = {
   publishers: [],
   plugins: [],
   hooks: { /* ... */ },
-  buildIdentifier: 'my-build'
+  buildIdentifier: 'my-build',
+  outDir: 'desired/outpath'
 };
 ```
 {% endtab %}
@@ -80,7 +81,8 @@ module.exports = {
       "publishers": [ ... ],
       "plugins": [ ... ],
       "hooks": { ... },
-      "buildIdentifier": "my-build"
+      "buildIdentifier": "my-build",
+      "outDir": "desired/outpath"
     }
   }
 }