Work on devenv.

doc/install.md

-# Install Instructions
+# Install Instructions for Ubuntu Machines
+
+Notice that ROS is only supported for Ubuntu LTS (that is 18.04 and
+20.04). We recommend to use 20.04.
 
 ## Virtual Machine with Ubuntu 18.04 or 20.04
 
+This section is instructions for if you run Ubuntu which is not 18.04 and 20.04.
+
 If you choose to use a virtual machine (not recommended if Gazebo
-should be used because of 2D graphics requirements) here are some
+should be used because of 3D graphics requirements) here are some
 information.
 
 ++++++++++++++++++ Which machine should this be done on?  What if you have a Windows machine as host?  ++++++++++++++++++