update quickstart, authenticator doc with 5.0 allow changes

docs/source/tutorial/quickstart.md

@@ -71,6 +71,35 @@ jupyterhub -h
 configurable-http-proxy -h
 ```
 
+## Configuration
+
+At this point, we could start jupyterhub, but nobody would be able to use it!
+Only users who are explicitly **allowed** can use JupyterHub.
+To allow users, we need to create a configuration file.
+JupyterHub uses a configuration file called `jupyterhub_config.py`,
+which is a regular Python script with one function `get_config()` pre-defined, returning the "config object".
+Assigning attributes to this object is how we configure JupyterHub.
+
+At this point, we have two choices:
+
+1. allow any user who can successfully login with our Authenticator (often a good choice for local machines with PAM)
+2. allow one or more users by name.
+
+We'll start with the first one.
+Create the file `jupyerhub_config.py` with the content:
+
+```python
+c = get_config()  # noqa
+c.Authenticator.allow_all = True
+# alternative: c.Authenticator.allowed_users = {"yourusername"}
+```
+
+This configuration means that anyone who can login with PAM (any existing user on the system) should have access to JupyterHub.
+
+:::{seealso}
+[](authenticators)
+:::
+
 ## Start the Hub server
 
 To start the Hub server, run the command: