Update README.md

Include mention of `data.table` R package for performant data frame manipulations.

README.md

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 #### Data Manipulation | Data Analysis | Data Visualization
 
+* [data.table](https://rdatatable.gitlab.io/data.table/) - `data.table` provides a high-performance version of base R’s `data.frame` with syntax and feature enhancements for ease of use, convenience and programming speed.
 * [dplyr](https://www.rdocumentation.org/packages/dplyr/versions/0.7.8) - A data manipulation package that helps to solve the most common data manipulation problems.
 * [ggplot2](https://ggplot2.tidyverse.org/) - A data visualization package based on the grammar of graphics.
 * [tmap](https://cran.r-project.org/web/packages/tmap/vignettes/tmap-getstarted.html) for visualizing geospatial data with static maps and [leaflet](https://rstudio.github.io/leaflet/) for interactive maps