readme: add tip about how to build in other languages

Author: nhsz

README.md

@@ -96,6 +96,7 @@ yarn dev
 - Pro Tip:
   - Explore scripts within `package.json` for more build options
   - Get **faster** production builds by building only one language. E.g. in your `.env` file, set `BUILD_LOCALES=en` to build the content only in English
+  - To build the site in other languages too, you need to set them in `BUILD_LOCALES`, eg: `` BUILD_LOCALES=`en,es` `` if you also want to include Spanish
 
 By default the script will build all the languages (complete list in `i18n.config.json`).