LEGAL: mention error_highlight

LEGAL

@@ -1017,6 +1017,17 @@ mentioned below.
 
     {MIT License}[rdoc-label:label-MIT+License]
 
+[lib/error_highlight]
+[lib/error_highlight.rb]
+[test/error_highlight]
+
+  error_highlight is under the following license.
+
+  >>>
+    Copyright (c) 2021 Yusuke Endoh
+
+    {MIT License}[rdoc-label:label-MIT+License]
+
 [benchmark/so_ackermann.rb]
 [benchmark/so_array.rb]
 [benchmark/so_binary_trees.rb]