added a link to the source code

I feel that this link is sufficient to meet the requirements of the AGPL,
provided that anyone who both modifies the webapp and arranges it to be
used remotely over the network by others (not a supported use case
currently!) updates the link to point to the full source code of their
modified version.

In particular, I think that putting this on the About page is a
prominant offer of the code as required by the AGPL.

templates/documentation/about.hamlet

@@ -20,6 +20,9 @@
       <a href="http://git-annex.branchable.com/license">
         this page.
     <p>
+    The source code for git-annex can be downloaded #
+    <a href="http://source.git-annex.branchable.com/">here</a>.
+    <p>
     Development of git-annex is made possible by #
     <a href="http://git-annex.branchable.com/assistant/thanks/">
       many excellent people