README: add documentation about the template tag

README

@@ -270,3 +270,16 @@ Template::
             </tr>
         {% endfor %}
     </table>
+
+Template tag
+============
+
+You can render a block in any template using the template tag ``render_blurp``:
+
+    {% load blurp_tags %}
+
+    {% render_blurp "student_table" %}
+
+Beware that it will not work with renderes doing any access to the
+``placeholder`` or ``instance`` parameter of their render method as those
+object are not available in a random template.