Added i18n blurb to docs

Added some info about i18n to docs.

@jonasll @fredryk any suggestions for more/less info? It links off to the docs page.
Timber leverages Shopify's translation engine to let any developer or merchant easily create their own theme language. <a href="http://docs.shopify.com/themes/theme-development/translations" class="docs-link">Learn more about theme internationalization</a>.