Add README examples

[chrismytton]

Add a README with some potential use-cases fleshed out.

Still to address:

• [x] Remove comments about term improvements, we should force people to use SQL+SPARQL
• [x] "have both both"
• [ ] Better section heading in Mediawiki output example? Currently: "Records matched to Wikidata but not returned by SPARQL query"
• [ ] Add a non-morph based example, e.g. Scotland
• [ ] Add an example of comparing pairs and triples, rather than just IDs, e.g. the Heads of Govenment scraper has 3 Wikidata IDs, and we want to compare on all three values.

[chrismytton]

This is quite out of date. I think we should add some code level documentation to the README instead, which is tracked by https://github.com/everypolitician/compare_with_wikidata/issues/44.