document i18n best practices

Description: We need to document our i18n plan of attack to make sure that translatable strings are handled consistently, namely: HTML in translated values appears to be the standard in the i18n world although react-intl recommends against using <FormattedHTMLMessage> due to security concerns, it appears to be our best option. We'll use it paired with a sanitizer such as https://github.com/punkave/sanitize-html to make sure we have clean output.

For context, see the two-day long discussion covering all the options and their tradeoffs: https://folio-project.slack.com/archives/C210UCHQ9/p1522155566000121