README: Document how to avoid Waffle with piglit

Suggested by Paulo.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/README b/README
index 246e24c..f108b20 100644
--- a/README
+++ b/README
@@ -40,6 +40,16 @@
 	piglit-sources $ cd bin
 	piglit-sources/bin $ ln $i-g-t-sources igt -s
 
+	To avoid some hassles with piglit's use of Waffle just disable it. Run
+
+	piglit-soruces $ cmake -D PIGLIT_USE_WAFFLE=OFF .
+
+	With
+
+	piglit-soruces $ ccmake .
+
+	you can check your configuration with a curses interface, too.
+
 	The tests in the i-g-t sources need to have been built already. Then we
 	can run the testcases with (as usual as root, no other drm clients
 	running):