Document write-mode options

[theduke]

It would be nice to have a table explaining the different write-modes in the README (and also in the --help output).

I'd do a PR but I need some help: what are the plain and coverage modes doing exactly?

write-mode	description
replace	replaces formatted files, creating backup filename.rs.bk files with the original content
overwrite	overwrites files without creating a backup
display	print full formatted content of each file
plain	???
diff	show diff of formatting changes
coverage	???
checkstyle	print information in checkstyle XML format

[csmoe]

rustfmt has move from write_mode to emit_mode since commit

checkout emit_mode:
https://github.com/rust-lang-nursery/rustfmt/blob/f390626778c1bbb13911556d585850eb2fa67923/src/config/options.rs#L172-L187
`

[bradjc]

For anyone finding this issue (like me) note that:

• --write-mode=overwrite is the default
• --write-mode=check is now --check
• --write-mode is now --emit

[ncharlton02]

I was thinking about tackling this. Should I include the info in both (readme and --help) or just under the help flag?

[bradjc]

I vote both.