Document user with limited rights

This patch adds documentation on how to add and use a user with limited
access rights for the display.

README.md

@@ -31,3 +31,28 @@ https://github.com/virtUOS/opencast-ca-display/assets/1008395/ead22cd2-9d7a-4d26
 - The display and laptop do not know about each other
 - The laptop is running an Opencast capture agent
 - When the laptop starts capturing video, the display shows an active recording
+
+## Opencast User
+
+To improve security, you can limit the access rights for the Opencast user by
+creating a user which has only read access to the capture agent status API and
+nothing else.
+
+To do this, first create a new security rule in your Opencast's
+`etc/security/mh_default_org.xml` allowing read access for a new role
+`ROLE_CAPTURE_AGENT_CALENDAR`:
+
+```xml
+<!-- Enable capture agent updates and ingest -->
+<sec:intercept-url pattern="/capture-admin/agents/**" method="GET" access="ROLE_ADMIN, ROLE_CAPTURE_AGENT, ROLE_CAPTURE_AGENT_CALENDAR" />
+<sec:intercept-url pattern="/capture-admin/**" access="ROLE_ADMIN, ROLE_CAPTURE_AGENT" />
+```
+
+Next, go to the Opencast  REST Docs → `/user-utils` and fill out the form for
+`POST /` with data like this:
+
+- username: `ca-display`
+- password: `secret-password`
+- roles: `["ROLE_CAPTURE_AGENT_CALENDAR"]`
+
+You should now be able to use this new user.