Update check_macros.h

ruy/check_macros.h

diff --git a/ruy/check_macros.h b/ruy/check_macros.h
index f77fd94..60fc577 100644
--- a/ruy/check_macros.h
+++ b/ruy/check_macros.h
@@ -13,6 +13,8 @@
 limitations under the License.
 ==============================================================================*/
 
+// self-contained, minimal, CHECK/DCHECK macros similar to glog.
+
 #ifndef RUY_RUY_CHECK_MACROS_H_
 #define RUY_RUY_CHECK_MACROS_H_