Add USAJOBS Help Center example in the readme

[afeijoo]

Update the readme to add the following example.

The 18F Glossary is used in the USAJOBS Help Center too, which you can see on competitive service at https://www.usajobs.gov/Help/working-in-government/appointments/examination/.

Source: https://gsa-tts.slack.com/archives/C3F14AHSQ/p1585226486012200?thread_ts=1584715110.163600&cid=C3F14AHSQ