Links to public data snapshots in data folder in README

[Lextuga007]

Would be really useful to have links to the IMD 2019 and GP practice July 2022 ICB in the README as these aren't always widely known and can help locate the original files. It's something I always struggle with 😃

[jacgrout]

• Add link for IMD 2019
• Add link / explanation for GP Practice to ICB