diff --git a/.gitignore b/.gitignore
index 56f9339..9aec231 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,6 +3,4 @@ flamegraph.svg
 perf.data
 perf.data.old
 
-# because env var is in here
-.cargo/config.toml
 data
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..7c918de
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "rapidjson"]
+	path = rapidjson
+	url = https://github.com/Tencent/rapidjson.git
diff --git a/README.md b/README.md
index 4638b0e..0af4934 100644
--- a/README.md
+++ b/README.md
@@ -22,8 +22,12 @@ See releases on the right of this repo page. Executables for linux, windows, mac
 
 If there's no executable to suit you, you'll need to build your own:
 
-Clone this repo.
+Clone this repo:
+```
+git clone --recurse-submodules --shallow-submodules https://github.com/djellemah/jch.git
+```
 
+Build it:
 ``` bash
 cargo build --release
 ```
@@ -91,3 +95,19 @@ Left hand column is the aggregated type of all leaves at the path in the right-h
 - `Xxx:nnnn` means `nnnn` was the number of values encountered, ie the number of leaf nodes matching the path.
 
 - If more than one type was encountered at the path, the left hand column will contain an array of characteristics, as above. That is, it's a sum type.
+
+# Advanced Build
+You can use an existing rapidjson tree by specifying the `RAPIDJSON_INCLUDE` env var.
+
+Either on the command line
+
+```
+RAPIDJSON_INCLUDE=your_source_dir cargo build
+```
+
+or in `.config/cargo.toml` like this
+
+```
+[env]
+RAPIDJSON_INCLUDE = "rapidjson/include"
+```
diff --git a/build.rs b/build.rs
index ff86b13..f9bf781 100644
--- a/build.rs
+++ b/build.rs
@@ -2,15 +2,14 @@ fn main() {
     let rapidjson_include = std::env::var("RAPIDJSON_INCLUDE");
     let rapidjson_include = match rapidjson_include {
         Ok(env_value) => std::path::PathBuf::from(env_value),
-        Err(err) => {
-            let msg = format!("\nCan't find RAPIDJSON_INCLUDE env var because {err:?}.\nYou can also set it in .cargo/config.toml under the [env] table.");
-            println!("{msg}");
-            std::process::exit(1)
-        }
+        Err(_) => "rapidjson/include".into(),
     };
 
     if !rapidjson_include.exists() {
-        println!("RAPIDJSON_INCLUDE value {} does not exist.", rapidjson_include.display());
+        println!(r#"
+            RAPIDJSON_INCLUDE env var path "{}" does not exist.
+            You can also set RAPIDJSON_INCLUDE it in .cargo/config.toml under the [env] table.
+        "#, rapidjson_include.display());
         std::process::exit(1)
     }
 
diff --git a/rapidjson b/rapidjson
new file mode 160000
index 0000000..ab1842a
--- /dev/null
+++ b/rapidjson
@@ -0,0 +1 @@
+Subproject commit ab1842a2dae061284c0a62dca1cc6d5e7e37e346