book: document splitting a project into crates

Author: Be-ing

book/src/concepts/build_systems.md

@@ -28,3 +28,18 @@ When using QML with CXX-Qt [QML modules](https://doc.qt.io/qt-6/qtqml-writing-a-
 This allows for attributes such as `#[qml_element]` to register the QObject with the QML type system without any C++ code.
 
 See [`QmlModule` documentation](https://docs.rs/cxx-qt-build/latest/cxx_qt_build/struct.QmlModule.html) for more details.
+
+## Splitting a project into multiple crates
+
+As your project grows, it can be helpful to organize your code into multiple Rust crates. If your `main` function is
+in Rust ([Cargo is your only build system](../getting-started/4-cargo-executable.md)), simply add the crates as
+dependencies of the top level binary crate in its Cargo.toml file.
+
+If your `main` function is in C++, you can only link one staticlib Rust crate into C++, otherwise linking
+would fail with duplicate symbol errors from multiple Rust runtimes. So, create one top level staticlib crate to link
+into the C++ application. Specify your other crates as normal Rust library (rlib) dependencies
+in the staticlib crate's Cargo.toml. You must reference the symbols of the Rust dependencies within the staticlib crate;
+if you don't need those symbols in Rust code, you can add `extern crate crate_name;` statements in the staticlib's lib.rs file.
+Refer to the [meta_project example](https://github.com/KDAB/cxx-qt/blob/main/examples/meta_project) for how to set this up.
+Note that this requires Rust compiler features that were [stabilized](https://github.com/rust-lang/rust/pull/113301)
+in Rust 1.74.