openpolicing.stanford.edu site is missing links to external datasets

[anshukla]

The README.md asks users to download census data and PPCS from the openpolicing.stanford.edu site. However, the data page doesn't have links to either dataset.

I've verified that the following links actually work:

https://stacks.stanford.edu/file/druid:py883nd2578/ppcs.tsv
https://stacks.stanford.edu/file/druid:py883nd2578/census-clean.csv.gz
https://stacks.stanford.edu/file/druid:py883nd2578/census-raw.csv.gz

which I was able to infer from the file paths of other data and constants in util/constants_and_paths.R

[janovergoor]

Thanks! We should put these links on the website also.