Adds a link to the text version of the BNF file. (#37)

Author: gkellogg

spec/index.html

@@ -642,6 +642,7 @@ <h3>Grammar</h3>
     </div>
 
     <div data-include="trig-bnf.html"></div>
+    <p>A text version of this grammar is available <a href="trig.bnf">here</a>.</p>
 
   </section>
   <section>