add biorxiv link

lib/imp.html

@@ -383,7 +383,7 @@
         <!-- REFERENCES -->
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-            <h2><small>Please cite the pre-print article: </small></h2>
+            <h2><small>Please cite the pre-print article on <a href="http://biorxiv.org/">http://biorxiv.org</a>.</small></h2>
             <div>
                 <p class="lead">
                 <strong>IMP: a pipeline for reproducible integrated metagenomic and metatranscriptomic analyses</strong><br/>