Update WASI-tutorial.md

docs/WASI-tutorial.md

@@ -77,9 +77,11 @@ which is configured to target WASI and use the WASI sysroot by default, so we ca
 compile our program like so:
 
 ```
-$ clang demo.c -o demo.wasm
+$ clang demo.c --sysroot <path to sysroot> -o demo.wasm
 ```
 
+The sysroot is folder is located in `opt/wasi-sdk/share/sysroot/`.
+
 A few things to note here. First, this is just regular clang, configured to use
 a WebAssembly target and sysroot. Second, the output name specified with the "-o"
 flag can be anything you want, and *does not* need to contain the `.wasm` extension.