add example using the ukpolice package

[njtierney]

Use the data from the ukpolice package to illustrate examples of maximum coverage.

[njtierney]

Their data portal might have some other useful data for building locations

[njtierney]

• Included york dataset in commit https://github.com/njtierney/maxcovr/commit/593a836e686d14de4d89f08fc109030f28f36913
• ukpolice data currently included in README, in commit https://github.com/njtierney/maxcovr/commit/0cf221df3ac474093f89b6bb3b67ee51b9f9053f