Added install_github instruction.

README.md

@@ -72,6 +72,11 @@ the default for a Mac:
 /Users/ldecicco/Library/R/2.15/library/dataRetrieval
 Then, re-install the package using the directions above.  Moving to CRAN should solve this problem.
 
+If you are an experienced R developer, with the devtools package and all associated LaTeX tools installed, the following command will install the dataRetrieval package directly from gitHub:
+
+	library(devtools)
+	install_github("dataRetrieval", "USGS-R")
+
 Background Information
 ----------------------