"Getting started" documentation and sample settings file need a few updates

[dhdaines]

The sample local_settings.py file is out of date in a few respects:

• It should probably include or at least mention the existence of CSRF_TRUSTED_ORIGINS as nothing will work without this! without which logging in as a local user (not via OAuth2) doesn't work.
• The backend for OSM social authentication is now social_core.backends.openstreetmap_oauth2.OpenStreetMapOAuth2 (the previous one has been removed from the most recent version of python-social-auth)
• It should be noted somewhere that when setting up OAuth2 for OSM, the necessary scope is (for some reason) "Read user preferences (read_prefs)" and not "Sign in using OpenStreetMap" as one might be inclined to believe.

I can provide a PR 😄

[yohanboniface]

I can provide a PR 😄

Yes, please! 👍