Correct homepage contents

- v2-update is preferred (i.e. `v2-` prefix)
- but it's actually broken, due still existing `cabal-install` bug in
  non-append repository updates
- Also add explicit link to haskell.gitlab.com

scripts/build-repo.sh

@@ -104,9 +104,11 @@ EOF
     packages from <a href="https://hackage.haskell.org/">Hackage</a> for use
     with <a href="https://gitlab.haskell.org/ghc/ghc">GHC</a> prereleases.
 
+	<p>The source of this package repository is at <a href="https://gitlab.haskell.org/ghc/head.hackage">gitlab.haskell.org/ghc/head.hackage</a>.
+
     <p>To use package repository with <code>cabal-install</code> add the following
     to your project's <code>cabal.project.local</code> and run
-    <code>cabal new-update</code>:
+	<code>cabal v2-update</code>: (consider using <code>scripts/head.hackage.sh update</code> as <code>v2-update</code> is broken, <a href="https://github.com/haskell/cabal/issues/5952">Cabal bug #5952</a>)
     <pre><code>
 $(cat repo/cabal.project.local)
     </code></pre>