USAGE.md: link to language tag documentaiton

manubot#81 (comment)

Author: dhimmel

USAGE.md

@@ -120,7 +120,7 @@ For guidance on what CSL JSON should be like for different document types, refer
 ## Manuscript metadata
 
 [`content/metadata.yaml`](content/metadata.yaml) contains manuscript metadata that gets passed through to Pandoc, via a [`yaml_metadata_block`](http://pandoc.org/MANUAL.html#extension-yaml_metadata_block).
-`metadata.yaml` should contain the manuscript `title`, `authors` list, `keywords`, and language (`lang`).
+`metadata.yaml` should contain the manuscript `title`, `authors` list, `keywords`, and `lang` ([language tag](https://www.w3.org/International/articles/language-tags/ "W3C: Language tags in HTML and XML")).
 Additional metadata, such as `date`, will automatically be created by the Manubot.
 Manubot uses the [timezone](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) specified in [`build.sh`](build/build.sh) for setting the manuscript's date.
 For example, setting the `TZ` environment variable to `Etc/UTC` directs the Manubot to use Coordinated Universal Time.