imx6-sabresd/readme: Add an entry for glmark benchmark

Add an entry to explain how to run the glmark benchmark.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

board/freescale/imx6-sabresd/readme.txt

@@ -82,4 +82,8 @@ Running Qt5 Cinematic Demo:
 # export QT_QPA_EGLFS_KMS_CONFIG=/root/sabresd.json
 # /usr/share/Qt5/CinematicExperience/Qt5_CinematicExperience
 
+Running gl2mark benchmark:
+
+# glmark2-es2-drm
+
 Enjoy!