Flesh out the README.txt for the new settings arrangement

envs/README.txt

 Transitional for moving to new settings scheme. 
 
 To use: 
-django-admin.py runserver --settings=settings2.dev --pythonpath="."
+  django-admin.py runserver --settings=envs.dev --pythonpath=.
+
+NOTE: Using manage.py will automatically run mitx/settings.py first, regardless
+of what you send it for an explicit --settings flag. It still works, but might
+have odd side effects. Using django-admin.py avoids that problem. 
+django-admin.py is installed by default when you install Django.
+