Add a description about the relationship between empires and continents

[stepps00]

As a post https://github.com/whosonfirst-data/whosonfirst-data/issues/763 follow-up, update the README file to include details about the relationship between empire records and continent records.

In short, instead of appending an empire's hierarchy to a descendant record's hierarchy, the following should occur:

• Append the empire's wof:id to the descendant's hierarchy
• Lookup at the continent placetype
• Append the continent's hierarchy to the descendant