add installation dir template var

Author: max-tet

docs/developer_docs/includes/template_vars_fs.md

@@ -1,5 +1,6 @@
-| variable          | description                                        | example                                           |
-|-------------------|----------------------------------------------------|---------------------------------------------------|
-| `fs.app_data`     | The absolute path to your app's directory          | `/home/user/.freeshard/user_data/app_data/my-app` |
-| `fs.all_app_data` | The absolute path to the app data parent directory | `/home/user/.freeshard/user_data/app_data`                 |
-| `fs.shared`       | The absolute path to the directory for shared data | `/home/user/.freeshard/user_data/shared`                   |
+| variable          | description                                                           | example                                            |
+|-------------------|-----------------------------------------------------------------------|----------------------------------------------------|
+| `fs.app_data`     | The absolute path to your app's directory                             | `/home/user/.freeshard/user_data/app_data/my-app`  |
+| `fs.all_app_data` | The absolute path to the app data parent directory                    | `/home/user/.freeshard/user_data/app_data`         |
+| `fs.shared`       | The absolute path to the directory for shared data                    | `/home/user/.freeshard/user_data/shared`           |
+| `fs.installation_dir`       | The absolute path to the directory where your installation files live | `/home/user/.freeshard/core/installed_apps/my-app` |