Update README.md

README.md

@@ -40,7 +40,7 @@ Package Installation
 ---------------------------------
 While the dataRetreival package is in development (and not on CRAN), the zoo package must first be manually installed. To install the dataRetrieval package, you must be using R 3.0 or greater and run the following commands:
 
-	install.packages(c("zoo","XML","RCurl))
+	install.packages(c("zoo","XML","RCurl","plyr"))
 	install.packages("dataRetrieval", repos="http://usgs-r.github.com")