Added option in the build of the hello world example.
Without it there will be linker's errors.
In the demo in fact this option has been added.
Newcomers could struggle when starting the first example and leave.
@@ -92,7 +92,7 @@ fn main() {
The final step is building the application as follows:
```sh
-cargo build -Z build-std=std,core,alloc,panic_abort --target x86_64-unknown-hermit
+cargo build -Z build-std=std,core,alloc,panic_abort -Z build-std-features=compiler-builtins-mem --target x86_64-unknown-hermit
```
(You can set an easy alias for this in the `.cargo/config` file. Take a look at the [demo](https://github.com/hermitcore/rusty-demo/blob/master/.cargo/config))
