Add installation instructions.

Author: hadley

README.md

@@ -2,3 +2,12 @@ Rcpp
 ====
 
 [![Build Status](https://travis-ci.org/RcppCore/Rcpp.png)](https://travis-ci.org/RcppCore/Rcpp)
+
+## Installation
+
+To install Rcpp from github, first make sure you have a [package development environment](http://www.rstudio.com/ide/docs/packages/prerequisites), and then:
+
+```R
+install.packages("devtools")
+devtools::install_github("RcppCore/Rcpp")
+```