Add submit button to style guide

 <p>The canopy theme uses the 16-bit <a
 href=https://ethanschoonover.com/solarized>solarized</a> palette.</p>
 
+<h3>Forms</h3>
+
+<h4>Buttons</h4>
+<p>Submit buttons are green. They brighten on hover.</p>
+<button type=submit>Submit</button>
+
 <h2>Keyboard Bindings</h2>
 
 <dl>