doc: highlight the need to filter node ids

doc/userguide.md

@@ -69,10 +69,15 @@ cargo install radicle-native-ci --locked
 ~~~
 
 * Create a configuration file. You can call the file anything you like
-  (the example below assumes `ci-broker.yaml`). You should ensure the
-  Radicle node below is your normal node. You should make sure the
-  `report_dir` field points to a directory that exists. The example
-  below assumes `_rad` user; adjust paths as necessary.
+  (the example below assumes `ci-broker.yaml`). The example below
+  assumes `_rad` user; adjust paths as necessary. You should make sure
+  the `report_dir` field points to a directory that exists.
+
+  **IMPORTANT**: You should ensure the Radicle node below (identified
+  by the node id after `!Node`) is a node you trust, otherwise you
+  could end up running arbitrary code through a random patch created
+  by anyone. If your run several nodes, you can list them all within
+  an `!Or` expression.
 
 ~~~yaml
 db: /home/_rad/ci-broker.db
@@ -227,7 +232,7 @@ log: /home/_rad/radicle-ci-ambient.log
 base_url: "https://ci0.liw.fi//ambient-log"
 ~~~
 
-* Create `/srv/http` with write permission for your user:  
+* Create `/srv/http` with write permission for your user:
   `sudo install -d /srv/http -o $USER -g $USER`
 
 * Start the CI broker in the foreground:
@@ -476,7 +481,7 @@ ways:
 * a global filter and a default adapter
 * a list of triggers, each of which has its own filter, and specifies
   an adapter to use
-  
+
 Example:
 
 ~~~yaml