Add example to Jenkins plugin for permissions

Signed-off-by: Tim Jacomb <21194782+timja@users.noreply.github.com>

.changeset/nasty-lies-laugh.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-jenkins-backend': patch
+---
+
+Add example on how to passthrough permissions

plugins/jenkins-backend/README.md

@@ -43,6 +43,7 @@ export default async function createPlugin(
       config: env.config,
       catalog,
     }),
+    permissions: env.permissions,
   });
 }
 ```