Add thumbnail of cheat sheet (#239)

* Add cheat sheet thumbnail

* Add thumbnail to end of usage section.
* Fixes #219

Author: batpigandme

README.Rmd

@@ -29,7 +29,7 @@ stringr is built on top of [stringi](https://github.com/gagolews/stringi), which
 
 ## Installation
 
-```R
+```r
 # Install the released version from CRAN:
 install.packages("stringr")
 
@@ -105,6 +105,10 @@ As well as regular expressions (the default), there are three other pattern matc
 * `coll()`: match human letters
 * `boundary()`: match boundaries
 
+
+<a href="https://github.com/rstudio/cheatsheets/blob/master/strings.pdf"><img src="man/figures/cheatsheet-thumb.png" align="right" dpi="600"/></a>  
+
+
 ## Compared to base R
 
 R provides a solid set of string operations, but because they have grown organically over time, they can be inconsistent and a little hard to learn. Additionally, they lag behind the string operations in other programming languages, so that some things that are easy to do in languages like Ruby or Python are rather hard to do in R. 

README.md

@@ -152,7 +152,10 @@ pattern matching engines:
 
   - `fixed()`: match exact bytes
   - `coll()`: match human letters
-  - `boundary()`: match boundaries
+  - `boundary()`: match
+boundaries
+
+<a href="https://github.com/rstudio/cheatsheets/blob/master/strings.pdf"><img src="man/figures/cheatsheet-thumb.png" align="right" dpi="600"/></a>
 
 ## Compared to base R