diff --git a/README.md b/README.md
index 273691b..44a7a5d 100644
--- a/README.md
+++ b/README.md
@@ -429,6 +429,9 @@ Variables are set with the `Vars{"key": "value", ...}` option. Undefined
 variable references in the grammar will result in an error at construction
 time.
 
+Variables can also be set via the `set:"K=V"` tag. In this case, those variables will be available for that
+node and all children. This is useful for composition by allowing the same struct to be reused.
+
 When interpolating into flag or argument help strings, some extra variables
 are defined from the value itself: