[mathiasrw]

Closes #796

Stacked on #806.

Change

A new README section, "Hairy details", just before "Test". It is built on the layers of a run: each layer is optional, and works on what the layer before gives it, so the flags work together by their order.

• On the way in: the pattern (3 layers), the replacement (5 layers) and the files (7 layers), each with small examples that were run to get the output. The replacement shows how -R, --js and --replacement-file stack.
• On the way out: what is printed and where (a table of stdout and stderr), the delimiters and tails, --nl, the quiet and verbose flags, the status report, and the exit codes.
• Other details: --rename and --rename-path, what --output-match turns off, and when the files are handled one at a time.

The devguide has a short "Layers" part: use the layers to understand a run, to write tests, and to challenge new flags.

Tests

Only docs. yarn test-format passes.