message: Update I18n docs
  Add a default `formatStr` of `'PP'` (or any of its variants) to the example custom formatting function so that it defaults to whichever format the locale has for dates.  Show the example output without `formatStr` set.
diff: @@ -47,7 +47,9 @@ import { enGB, eo, ru } from 'date-fns/locale'
  const locales = {enGB, eo, ru}
  -export default function (date, formatStr) {
  +// by providing a default string of 'PP' or any of its variants for `formatStr`
  +// it will format dates in whichever way is appropriate to the locale
  +export default function (date, formatStr = 'PP') {
  return format(date, formatStr, {
  locale: locales[window.__localeId__] // or global.__localeId__
  })
  @@ -64,6 +66,16 @@ format(friday13, 'EEEE d')
  window.__localeId__ = 'eo'
  format(friday13, 'EEEE d')
  //=> 'vendredo 13'
  +
  +// If the format string is omitted, it will take the default for the locale.
  +window.__localeId__ = 'en'
  +format(friday13)
  +//=> Jul 13, 2019
  +
  +window.__localeId__ = 'eo'
  +format(friday13)
  +//=> 2019-jul-13
  +
  ```
  ## Adding New Language
  