Add shortcut for installation for developers

Author: Serene-Arc

CONTRIBUTING.rst

@@ -112,7 +112,7 @@ This can be done with:
 This will install ``beets`` and all development dependencies into its own
 virtual environment in your ``$POETRY_CACHE_DIR``. See ``poetry install --help``
 for installation options, including installing ``extra`` dependencies for
-plugins.
+plugins. If you want to install all extras, use ``poetry install --all-extras``.
 
 In order to run something within this virtual environment, start the command
 with ``poetry run`` to them, for example ``poetry run pytest``.