small guideline modif: not undirected graphs

guidelines.html

@@ -60,6 +60,7 @@ <h1>Guidelines</h1>
             <h2>In a nutshell</h2>
             <p>Our idea is to create an open source collection of <b>historical text genealogies</b>, in
               forms of tree-like graphs (also called <i>stemma</i>), for <b>any <!--european--> language</b>.</p>
+            <p>N.B.: only directed stemmata are considered, and undirected graphs are left out of the scope of our database (at least for now).</p>
             <p>Trees are encoded with the<a href="https://en.wikipedia.org/wiki/DOT_(graph_description_language)" target="_blank">
               DOT graph description language</a>: the image on the right is created out of the code on the left.</p>