Document Grattan's (Hugh) use of data.table

[mattdowle]

Awesome work by @HughParsonage. There's reference to things you can do in data.table that you can't do elsewhere, and 2 seconds vs 30 mins.
Would be great to do a proper benchmark and write up where data.table is being used to its fullest and compare to alternatives like dplyr and SQL. It could be a page on the wiki, or an article, or both. It could be linked from our homepage.
https://www.msn.com/en-gb/finance/other/coalitions-tax-plan-more-than-half-of-cuts-will-go-to-wealthiest-20-25-analysis-finds/ar-AAx3bzK
https://twitter.com/MattDowle/status/994637549023838208
https://twitter.com/hughparsonage/status/994639695819653120
https://twitter.com/hughparsonage/status/994640283697496068
https://github.com/HughParsonage/grattan/blob/master/R/income_tax.R

[HughParsonage]

@mattdowle Would something like https://hughparsonage.github.io/grattan/articles/datatable-implementation.html be a useful reference? Kept it short and to the point.