README: Expand on steps needed after the initial installation

Explain how to set up SSH for the "critic" user so that it can access
upstream repositories properly.

Add a mention of the "criticctl" tool, which is used to add users to the
Critic user database.

Change "account" to "system account" under "Adding push rights", to
differentiate with the Critic user account mentioned in the paragraph
just above.

README.md

@@ -30,12 +30,48 @@ as top level menu items, in addition to the usual menu items.  To add
 a new repository, click the 'Repositories' menu item and then the
 'Add Repository' button in the top right corner.
 
+If Critic only has ssh access to the upstream of your repository, you must
+set up the 'critic' system user (or whatever user name was chosen during
+installation) to have ssh access without the need of a password.  You can do
+that by creating an ssh key without a password and using 'ssh-copy-id' to
+copy the key across to the server.  If you need to connect to the upstream
+server using a different user name, you need to create a 'config' file in
+the 'critic' system user's '.ssh' directory containing:
+
+    Host <upstream-host-name>
+    User <upstream-user-name>
+
+Make sure to verify that you can access the repository from the 'critic'
+user by running something like this:
+
+    su -s /bin/bash -c "ssh -v <upstream-host-name>" critic
+
+This should also ensure that the upstream server key is stored in the
+'critic' user's 'known_hosts' file.
+
+This needs only to be done once per upstream server.
+
+Adding users
+------------
+
+If you are using the 'host' authentication system, users authenticated by
+the web server will be added automatically to the Critic user database.  If
+you are using the 'critic' authentication system you can use the 'critcctl'
+tool to add users beyond the administrative user created by the install.py
+script:
+
+    sudo criticctl adduser
+
+Only the users added with this method will be able to sign in to the system
+when using the 'critic' authentication system.
+
 Adding push rights
 ------------------
 
 Before a user can push review branches to the newly created Critic repository
-their account must be a member of the 'critic' system group (or whatever group
-name was chosen during installation).  In Debian/Ubuntu this can be done using:
+their system account must be a member of the 'critic' system group (or whatever
+group name was chosen during installation).  In Debian/Ubuntu this can be done
+using:
 
     usermod -a -G critic <user-who-wants-push-rights>