add stdin option on README

[tgfjt]

FYI it differs a little from actual log.
seems to be a issue of commandpost?

  Usage: tsfmt [options] [--] [files...]

  Options:

    -r, --replace      replace .ts file
    --stdin            get formatting content from stdin
    --no-tslint        don't read a tslint.json
    --no-editorconfig  don't read a .editorconfig
    --no-tsfmt         don't read a tsfmt.json
    --verbose          makes output more verbose

[vvakame]

@tgfjt thanks! it's my laziness. 😣