mesa: docs: Add link to planet.freedesktop.org

The freedesktop.org blog feeds aren't mentioned on either mesa3d.org or
any of the graphics project wikis (including the DRI wiki) on
freedeskop.org.  Fix that by linking to it from the sidebar.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
diff --git a/docs/contents.html b/docs/contents.html
index 5e0f514..294804f 100644
--- a/docs/contents.html
+++ b/docs/contents.html
@@ -90,6 +90,7 @@
 <li><a href="http://www.opengl.org" target="_parent">OpenGL website</a>
 <li><a href="http://dri.freedesktop.org" target="_parent">DRI website</a>
 <li><a href="http://www.freedesktop.org" target="_parent">freedesktop.org</a>
+<li><a href="http://planet.freedesktop.org" target="_parent">Developer blogs</a>
 </ul>
 
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