Add build script part to FFI chapter for more clear and smooth learn experience

src/ffi.md

@@ -21,6 +21,29 @@ libc = "0.2.0"
 
 [libc]: https://crates.io/crates/libc
 
+## Prepare the build script
+
+Because [snappy](https://github.com/google/snappy) is a static library by default. So there is no C++ std linked in the output artifact. In order to use this foreign library in Rust, we have to manually specify that we want to link stdc++ in our project. The easiest way to do this is by setting up a build script.
+
+First edit `Cargo.toml`, inside `package` add `build = "build.rs"`.
+```toml
+[package]
+...
+build = "build.rs"
+```
+
+Then create a new file at the root of your workspace, named `build.rs`.
+```rust
+// build.rs
+fn main() {
+    println!("cargo:rustc-link-lib=dylib=stdc++");
+    println!("cargo:rustc-link-search=<YOUR SNAPPY LIBRARY PATH>");
+}
+```
+
+For more information, please read [The Cargo Book - build script](https://doc.rust-lang.org/cargo/reference/build-scripts.html).
+
+
 ## Calling foreign functions
 
 The following is a minimal example of calling a foreign function which will