Add anchor for user-visible change

Author: philderbeast

CONTRIBUTING.md

@@ -292,7 +292,7 @@ Changelog
 
 Anything that changes `cabal-install:exe:cabal` or changes exports from library
 modules or changes behaviour of functions exported from packages published to
-hackage is a user-visible change.
+hackage is a <a id="user-visible-change">user-visible change</a>.
 
 When opening a pull request with a user-visible change, you should write one
 changelog entry (or more in case of multiple independent changes) — the