Add antifeatures array to template

data/plugins/template.yaml

@@ -8,5 +8,9 @@ license:
 author:
 # A short description for the plugin. May contain markdown.
 description:
+# Antifeature identifiers. Currently the only defined value is `synchronous`,
+# which must be included if the plugin uses non-async libraries in the main
+# thread (because that can block the whole maubot process).
+antifeatures: []
 # List of public bot user IDs where the plugin is running.
 public_instances: []