Allow easier installation for R package via devtools (#177)

* Fix to avoid issue #176 for R package installation

* Provide installation step using devtools

R-package/README.md

@@ -4,14 +4,39 @@ LightGBM R Package
 Installation
 ------------
 
+Windows users may need to run with administrator rights (either R or the command prompt, depending on the way you are installing this package). Rtools must be installed.
+
+You can use a command prompt to install via command line:
+
 ```
 cd R-package
 R CMD INSTALL --build  .
 ```
 
-For windows user, you may need to run command prompt as administrator.
+You can also install directly from R using the repository with `devtools`:
+
+```r
+devtools::install_github("Microsoft/LightGBM", subdir = "R-package")
+```
+
+To install LightGBM from a specific commit, you can specify the reference, such as the following to install the first release of the R package for LightGBM:
+
+```r
+devtools::install_github("Microsoft/LightGBM", ref = "1b7643b", subdir = "R-package")
+```
+
+You can check quickly if your LightGBM R package is working by running the following:
+
+```r
+library(lightgbm)
+data(agaricus.train, package='lightgbm')
+train <- agaricus.train
+dtrain <- lgb.Dataset(train$data, label=train$label)
+params <- list(objective="regression", metric="l2")
+model <- lgb.cv(params, dtrain, 10, nfold=5, min_data=1, learning_rate=1, early_stopping_rounds=10)
+```
 
 Examples
---------
+------------
 
 * Please visit [demo](demo).