[melodic] Update docs for ros2 branches

[chapulina]

🎉 New feature

• Part of Branch off for Galactic #200

Summary

Just a documentation update. It's important to update the melodic README because that's the first one people see when they go to https://github.com/ignitionrobotics/ros_ign (maybe we should change that)

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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