add more docs to __init__.py

xontrib/pygitstatus/__init__.py

@@ -1 +1,4 @@
-# This module is loaded by xontrib if no entrypoint is defined. -- https://xon.sh/tutorial_xontrib.html#structure
+# Xonsh used to use namespace packages as the only supported way to load xontribs, there is still code in older
+# versions of Xonsh that sort of expect this.
+# When using namespace packages this __init__.py file is loaded if no entrypoint is defined.
+#   -- https://xon.sh/tutorial_xontrib.html#structure