docs: add news notes to 18.1.0

CC: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>

docs/index.html

@@ -16,6 +16,13 @@
 
 <h1>News</h1>
 
+<h2>May 18, 2018</h2>
+<p>
+<a href="relnotes/18.1.0.html">Mesa 18.1.0</a> is released.  This is a
+new development release.  See the release notes for more information
+about the release.
+</p>
+
 <h2>May 17, 2018</h2>
 <p>
 <a href="relnotes/18.0.4.html">Mesa 18.0.4</a> is released.