[R-package] [docs] Added documentation on lgb.dl() and data.table ver…

…sion

R-package/README.md

@@ -1,6 +1,13 @@
 LightGBM R-package
 ==================
 
+### Contents
+
+* [Installation](#installation)
+* [Examples](#examples)
+* [Testing](#testing)
+* [External Repositories](#external-unofficial-repositories)
+
 Installation
 ------------
 
@@ -156,3 +163,10 @@ External (Unofficial) Repositories
 Projects listed here are not maintained or endorsed by the `LightGBM` development team, but may offer some features currently missing from the main R package.
 
 * [lightgbm.py](https://github.com/kapsner/lightgbm.py): This R package offers a wrapper built with `reticulate`, a package used to call Python code from R. If you are comfortable with the added installation complexity of installing `lightgbm`'s Python package and the performance cost of passing data between R and Python, you might find that this package offers some features that are not yet available in the native `lightgbm` R package.
+
+Known Issues
+------------
+
+## `error in data.table::data.table()...argument 2 is NULL`
+
+If you experiencing this error when running `lightgbm`, you may be facing the same issue reported in [#2715](https://github.com/microsoft/LightGBM/issues/2715). If you use `lgb.dl()` to build from source  (i.e. not using pre-compiled dll), you need to upgrade your version of `data.table` to at least  version 1.12.0.