fix: #1054 document the {filename}_meta.json companion file

[RajanChavada]

Fixes #1054. save_output() always writes a {filename}_meta.json companion
file next to the primary output for every --output_format, which surprised
users who read it as a bug. This documents the behavior in the marker_single
section of the README (no behavior change).

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[RajanChavada]

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