Modified launch files tutorial, added a note to the README.

[Jakeisthesnake]

Description

Launch File tutorial
Added explanations for the member function of MoveItConfigsBuilder.
Added note about launch nodes for custom .cpp files.
Having made these suggestions, I now wonder if I am being too verbose and just cluttering up the documentation when someone wanting to learn more can do what I did and read the source code. So maybe the launch file tutorial should remain unchanged.

README
Added a note that building MoveIt can be skipped if only adding explanations, note any executable code. This is mostly addressing my own pain of building MoveIt (about a 3-4 hour process) and I could see it stopping someone from adding a few helpful comments.

Please explain the changes you made, including a reference to the related issue if applicable

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