Add notice about YWSD in docs

docs/reference/ywsd.md

@@ -0,0 +1,10 @@
+# YWSD
+
+FieldPOC is using [Yate Wähl System Digital](https://github.com/eventphone/ywsd) for call routing.
+
+The routes are stored in a PostgreSQL database that is populated by FieldPOC.
+Actual routing is handled by the YWSD software that is started as part of FieldPOC.
+
+When FieldPOC stopped, YWSD is stopped to.
+Therefore running calls will continue as long as everything else is working.
+But new calls can not be established.

mkdocs.yml

@@ -18,6 +18,7 @@ nav:
     - reference/configuration.md
     - reference/extensions.md
     - reference/controller.md
+    - reference/ywsd.md
   - Operation:
     - operation/claim-dect-extension.md
     - operation/troubleshooting.md