docs: replace URL in features.txt

Replace unmaintained http://dri.freedesktop.org/wiki/MissingFunctionality
URL with http://mesamatrix.net/

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95460
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
diff --git a/docs/features.txt b/docs/features.txt
index 55b1fbb..2f2d41d 100644
--- a/docs/features.txt
+++ b/docs/features.txt
@@ -333,5 +333,6 @@
   GL_ARB_shadow_ambient                                 Superseded by GL_ARB_fragment_program
   GL_ARB_vertex_blend                                   Superseded by GL_ARB_vertex_program
 
-More info about these features and the work involved can be found at
-http://dri.freedesktop.org/wiki/MissingFunctionality
+
+A graphical representation of this information can be found at
+http://mesamatrix.net/