docs: Add section explaining GitHub/Google auth for dev env.

docs/settings.md

@@ -108,6 +108,47 @@ maximize convenience of testing all of Zulip's features; they should
 be enabled by default in production if we expect most Zulip sites to
 want those settings.
 
+#### Testing Google & GitHub authentication
+
+In order to set up Google or GitHub authentication in the development
+environment, you'll have to go through the steps detailed in
+`prod_settings_template.py` with some changes. Here is the full
+procedure:
+
+##### Google
+
+* Visit https://console.developers.google.com, click on Credentials on
+the left sidebar and create a Oauth2 client ID that allows redirects
+to `https://localhost:9991/accounts/login/google/done/`.
+
+* Go to the Library (left sidebar), then under "Social APIs" click on
+"Google+ API" and click the button to enable the API.
+
+* Uncomment `'zproject.backends.GoogleMobileOauth2Backend'` in
+`AUTHENTICATION_BACKENDS` in `dev_settings.py`.
+
+* Uncomment `GOOGLE_OAUTH2_CLIENT_ID` in `prod_settings_template.py` &
+assign it the Client ID you got from Google.
+
+* Put the Client Secret you got from Google as
+`google_oauth2_client_secret` in `dev-secrets.conf`.
+
+##### GitHub
+
+* Register an OAuth2 application with GitHub at one of
+https://github.com/settings/developers or
+https://github.com/organizations/ORGNAME/settings/developers.
+Specify `http://localhost:9991/complete/github/` as the callback URL.
+
+* Uncomment `'zproject.backends.GitHubAuthBackend'` in
+`AUTHENTICATION_BACKENDS` in `dev_settings.py`.
+
+* Uncomment `SOCIAL_AUTH_GITHUB_KEY` in `prod_settings_template.py` &
+assign it the Client ID you got from GitHub.
+
+* Put the Client Secret you got from GitHub as
+`social_auth_github_secret` in `dev-secrets.conf`.
+
 #### Testing non-default settings
 
 You can write tests for settings using e.g. `with