Add a code example to README.md

Author: jannic

README.md

@@ -12,7 +12,37 @@ This project is developed and maintained by the [Cortex-M team][team].
 
 ## Example
 
-For a usage example, see `examples/global_alloc.rs`.
+Starting with Rust 1.68, this crate can be used as a global allocator:
+
+```
+#![no_std]
+#![no_main]
+
+extern crate alloc;
+
+use cortex_m_rt::entry;
+use embedded_alloc::Heap;
+
+#[global_allocator]
+static HEAP: Heap = Heap::empty();
+
+#[entry]
+fn main() -> ! {
+    // Initialize the allocator BEFORE you use it
+    {
+        use core::mem::MaybeUninit;
+        const HEAP_SIZE: usize = 1024;
+        static mut HEAP_MEM: [MaybeUninit<u8>; HEAP_SIZE] = [MaybeUninit::uninit(); HEAP_SIZE];
+        unsafe { HEAP.init(HEAP_MEM.as_ptr() as usize, HEAP_SIZE) }
+    }
+
+    // now the allocator is ready types like Box, Vec can be used.
+
+    loop { /* .. */ }
+}
+```
+
+For a full usage example, see `examples/global_alloc.rs`.
 
 ## [Documentation](https://docs.rs/embedded-alloc)