added a note on /opt/srlinux/models/srl_nokia source-directory

Author: hellt

docs/ndk/guide/agent.md

@@ -76,6 +76,8 @@ greeter:
 
 7. The source directories where to search for the YANG modules. The `/opt/greeter/yang` directory should contain a YANG module with the `greeter` name.
 
+    If your agent imports any existing SR Linux YANG modules, you should add the `/opt/srlinux/models/srl_nokia` directory to the list of source directories.
+
 ///details | Complete list of config files parameters
 
 ```yaml