style-guide: Move and expand text about trailing commas

`principles.md` includes some high-level guiding principles for
formatting, but also includes a few specific formatting provisions.
While those provisions apply in many places, the same holds true for
other high-level guidance. Move the text about trailing commas to
`README.md`, so that `principles.md` can focus on guiding principles
while the top level of the style guide gives concrete formatting
recommendations.

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