Add example showing how to specify local authority

[Robinlovelace]

E.g.

remotes::install_github("cyipt/acton")
#> Skipping install of 'acton' from a github remote, the SHA1 (732457a5) has not changed since last install.
#>   Use `force = TRUE` to force installation
library(acton)
res = acton::get_planit_data(auth = "leeds")
#> Getting data from https://dev.planit.org.uk/api/applics/geojson?limit=6&bbox=&end_date=2020-03-08&start_date=2000-02-01&pg_sz=6&auth=leeds
mapview::mapview(res)

^{Created on 2020-03-08 by the reprex package (v0.3.0)}