add description

[Former user]

Hi, I'm a new user of using Oxts 2500 under ROS.

I think it should be more better to add the coordinate description from product view to ROS view in readme or something else, with picture is much pretty good.

because ROS is ENU standard. new users don't know what the output data is in if didn't check the issue list.

[Kevin Hallenbeck]

The README was updated to say East/North/Up for x/y/z in commit 577dee5