Add cheat sheet thumbnail to README (#505)

Author: batpigandme

README.Rmd

@@ -39,6 +39,10 @@ install.packages("purrr")
 devtools::install_github("tidyverse/purrr")
 ```
 
+## Cheatsheet
+
+<a href="https://github.com/rstudio/cheatsheets/blob/master/purrr.pdf"><img src="https://raw.githubusercontent.com/rstudio/cheatsheets/master/pngs/thumbnails/purrr-cheatsheet-thumbs.png" width="630" height="252"/></a>  
+
 ## Usage
 
 The following example uses purrr to solve a fairly realistic problem: split a data frame into pieces, fit a model to each piece, compute the summary, then extract the R^2^.

README.md

@@ -33,6 +33,10 @@ install.packages("purrr")
 devtools::install_github("tidyverse/purrr")
 ```
 
+## Cheatsheet
+
+<a href="https://github.com/rstudio/cheatsheets/blob/master/purrr.pdf"><img src="https://raw.githubusercontent.com/rstudio/cheatsheets/master/pngs/thumbnails/purrr-cheatsheet-thumbs.png" width="630" height="252"/></a>
+
 ## Usage
 
 The following example uses purrr to solve a fairly realistic problem: