Add a short usage section to Readme

Author: phil-opp

README.md

@@ -4,5 +4,29 @@
 
 [Documentation](http://phil-opp.github.io/linked-list-allocator/linked_list_allocator/index.html)
 
+## Usage
+
+Create a static allocator in your root module:
+
+```rust
+use linked_list_allocator::LockedHeap;
+
+#[global_allocator]
+static ALLOCATOR: LockedHeap = LockedHeap::empty();
+```
+
+Before using this allocator, you need to init it:
+
+```rust
+pub fn init_heap() {
+    let heap_start = …;
+    let heap_end = …;
+    let heap_size = heap_end - heap_start;
+    unsafe {
+        ALLOCATOR.lock().init(heap_start, heap_size);
+    }
+}
+```
+
 ## License
 This crate is dual-licensed under MIT or the Apache License (Version 2.0). See LICENSE-APACHE and LICENSE-MIT for details.