docs: add INTEL_conservative_rasterization to relaese notes for 13.1.0

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

docs/relnotes/13.1.0.html

@@ -45,6 +45,7 @@ Note: some of the new features are only available with certain drivers.
 
 <ul>
 <li>GL_ARB_post_depth_coverage on i965/gen9+</li>
+<li>GL_INTEL_conservative_rasterization on i965/gen9+</li>
 <li>GL_NV_image_formats on any driver supporting GL_ARB_shader_image_load_store (i965, nvc0, radeonsi, softpipe)</li>
 </ul>