Add TNA UK API examples

[wragge]

Post about the API: https://blog.nationalarchives.gov.uk/blog/using-the-discovery-api/

[DavidUnderdown]

A few of us are currently trying to develop wrapper modules for the various API endpoints in our learning and development time - so they'll be semi-officialish. Currently in a repo on my own Github account https://github.com/DavidUnderdown/TnaDiscoveryAPIv1