Add description of hashes

index.html

@@ -96,7 +96,7 @@
   </section>
   <footer>
     <a href="mailto:a.herten@fz-juelich.de" rel="author">—Andreas Herten</a>
-    <span class="git-hash">REPO_HASH // PAGES_HASH</span>
+    <span class="git-hash"><abbr title="Git hash of main repository">REPO_HASH</abbr> // <abbr title="Git hash of this page's branch">PAGES_HASH</abbr></span>
   </footer>
 </body>
 </html>