Update README to include more context

[francisbarton]

• Differences between ONSPD and NHSPD
• Features of the above including update schedule and whether they contain retired postcodes or just current
• How to use package functions to query the postcodes.io API
• How to use package functions to suggest potentially "corrected" postcodes for datasets that have invalid codes
• Other ways to access the NHSPD eg via the NHS API, as well as ONS OpenGeography. Different formats.
• How to solve common problems

[Lextuga007]

For {NHSRpopulation} we used pkgdown to build a site and if you call the vignette the same name as the repo and _pkgdown.yml has intro listed it sets up a Get Started vignette with its own url along the top. That might be better than a longer README. I've linked to the vignettes in the {NHSRpopulation} README for the benefit of new users who aren't aware of the link in the top right of the repository in GitHub.