diff --git a/Build.md b/Build.md
index c8b130b..7a3ba72 100644
--- a/Build.md
+++ b/Build.md
@@ -86,6 +86,13 @@ for your system, as well as methods of obtaining and locating Clang+LLVM.
   Specify paths where CMake can locate Clang/LLVM headers and libraries.
   CMake searches the paths in `CMAKE_PREFIX_PATH` for `include` and `lib`.
 
+* `-DUSE_SYSTEM_RAPIDJSON=(ON|OFF)`
+
+  Default: **ON**
+
+  Use system rapidjson (usually available at `/usr/include/rapidjson`).
+  If **OFF** or not found, `third_party/rapidjson` will be used instead.
+
 ## Choosing Clang+LLVM
 
 ### System Clang+LLVM