add NEWS link

Author: stevengj

NEWS.md

@@ -43,7 +43,7 @@ Language changes
 
   * Identifiers can now start with numeric symbols in category
     [No: Number, other](http://www.fileformat.info/info/unicode/category/No/list.htm),
-    allowing you to have variables with names like `⅓` or `¹x₂`.
+    allowing you to have variables with names like `⅓` or `¹x₂` ([#20278]).
 
 Breaking changes
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