我已经为 R 安装了devtools，但是当我使用以下命令调用库时：
library(devtools)
我得到以下输出：
WARNING: Rtools is required to build R packages, but is not currently installed.
Please download and install Rtools 3.1 from http://cran.r-project.org/bin/windows/Rtools/ and then run find_rtools().
所以我执行了以下代码：
install.packages("Rtools", repos = "http://cran.r-project.org/bin/windows/Rtools/")
我得到以下输出：
Installing package into ‘C:~/Documents/R/win-library/3.0’
(as ‘lib’ is unspecified)
Warning: unable to access index for repository http://cran.r-project.org/bin/windows/Rtools/bin/windows/contrib/3.0
Warning message:
package ‘Rtools’ is not available (for R version 3.0.2)
我将不胜感激任何帮助将 Rtools 安装到我的机器上，以便我可以调用 devtools 库。谢谢！