add more comments on .rosinstall

add. c.f to jsk-ros-pkg#1153

jsk_fetch_robot/jsk_fetch.rosinstall

@@ -72,6 +72,8 @@
     uri: https://github.com/ros-drivers/joystick_drivers.git
     version: 1.14.0
 # indigo is already EOL and fetch_ros is never released.
+# We cannot use version 0.8.x (for melodic) because some packages of fetch_ros cannot be built in indigo PC. This is because indigo PC uses OpenCV 2.4.8 while fetch_ros of 0.8.x requires OpenCV 3.2.
+# See https://github.com/jsk-ros-pkg/jsk_robot/pull/1148 and https://github.com/jsk-ros-pkg/jsk_robot/pull/1153
 - git:
     local-name: fetchrobotics/fetch_ros
     uri: https://github.com/fetchrobotics/fetch_ros.git