Add note about required-ros-distributions caveat on Ubuntu (#464)

* Add note about required-ros-distributions caveat on Ubuntu

Signed-off-by: Christophe Bedard <bedard.christophe@gmail.com>

README.md

@@ -139,6 +139,8 @@ build_docker:
     - run: "source /opt/ros/noetic/setup.bash && rosnode --help"
 ```
 
+**Note: on Ubuntu, `required-ros-distributions` installs the desktop variant for that distribution. This option is not required, and should probably be avoided in most workflows. It is retained for historical reasons and those who specifically do not care about whether their application specifies its dependencies properly.**
+
 ### Use pre-release ROS 2 binaries for testing
 
 You can specify if you'd like to use the [pre-release ROS 2 repository][pre_release_testing] in your sources list file by setting the `use-ros2-testing` parameter to `true`.

action.yml

@@ -20,8 +20,9 @@ inputs:
     description: |
       List of binary ROS distributions to be installed.
 
-      Support for non-Linux platform is not implemented yet, so this
-      parameter is ignored on Mac OS X and Windows.
+      On Ubuntu, this installs the desktop variant for the given distribution.
+      On Windows, this downloads and extracts the binaries for the given distribution.
+      This parameter is ignored on macOS.
 
       Allowed ROS distributions
       - "" (no value) - no ROS binary installation
@@ -36,8 +37,6 @@ inputs:
 
       Multiple values can be passed using a whitespace delimited list
       "melodic dashing".
-
-      For each passed ROS distribution, its desktop variant will be installed.
     required: false
     default: ""
 runs: