Add a minimal config.toml

Author: juntyr

src/doc/rustc/src/platform-support/wasm32-unknown-emscripten.md

@@ -59,11 +59,19 @@ source ./emsdk/emsdk_env.sh
 Building this target can be done by:
 
 * Configure the `wasm32-unknown-emscripten` target to get built.
-* Configure LLD to be built.
 * Ensure the `WebAssembly` target backend is not disabled in LLVM.
 
 These are all controlled through `config.toml` options. It should be possible
-to build this target on any platform.
+to build this target on any platform. A minimal example configuration would be:
+
+```toml
+[llvm]
+targets = "WebAssembly"
+
+[build]
+build-stage = 1
+target = ["wasm32-unknown-emscripten"]
+```
 
 ## Building Rust programs