Add docs for cross compile

CrossCompile.md

@@ -0,0 +1,52 @@
+# Cross Compile
+
+To do cross compile, you should set the toolchains correctly, espicially the target linker, please read the [Cargo target reference](https://doc.rust-lang.org/cargo/reference/config.html#target) first.
+
+Here are some examples for cross compile.
+
+## Build on macOS and target Linux
+
+1. Install the rustc toolchain for Linux
+
+```sh
+rustup target add x86_64-unknown-linux-gnu
+```
+
+2. Install a prebuilt cross compiler for Linux
+
+```sh
+brew tap SergioBenitez/osxct
+brew install x86_64-unknown-linux-gnu
+```
+
+3. Do cross compile:
+
+```sh
+CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER=x86_64-unknown-linux-gnu-gcc \
+cargo build --target=x86_64-unknown-linux-gnu
+```
+
+The `CARGO_TARGET_<triple>_LINKER` environment is used to specify the linker for cross compiling.
+
+## Build on macOS and target Android
+
+1. Install the rustc toolchain for Android
+
+```sh
+rustup target add arm-linux-androideabi aarch64-linux-android
+```
+
+2. Install ndk for Android, you can download it in [here](https://developer.android.com/ndk/downloads)
+3. Set the linker for cross compile:
+
+```sh
+CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER=$NDK/toolchains/llvm/prebuilt/darwin-x86_64/bin/aarch64-linux-android30-clang
+```
+
+the `$NDK` is the ndk installation directory.
+
+4. Do cross compile:
+
+```sh
+cargo build --target aarch64-linux-android
+```

README.md

@@ -21,6 +21,8 @@ You may also change the target architecture with the `--target` flag
 cargo build
 ```
 
+If you want to do cross compile, read [CrossCompile](./CrossCompile.md)
+
 ## Run
 
 ```bash