Update docs.

docs.md

@@ -280,6 +280,27 @@ pub struct Foo { .. }; // This won't be emitted by cbindgen in the header
 fn bar() -> Foo { .. } // Will be emitted as `struct foo bar();`
 ```
 
+### Volatile annotation
+
+cbindgen will emit the C volatile type qualifier for types and variables that have the `volatile=true` annotation.
+
+There is no equivalent in rust. You should use `read_volatile` and `write_volatile` to get C-like behavior.
+
+```rust
+/// cbindgen:volatile=true
+type volatile_type_t = u32;
+
+#[repr(C)]
+struct S {
+    /// cbindgen:volatile=true
+    volatile_variable: u32,
+}
+
+/// cbindgen:volatile=true
+#[no_mangle]
+static mut volatile_global_variable: u32 = 0;
+```
+
 ### Struct Annotations
 
 * field-names=\[field1, field2, ...\] -- sets the names of all the fields in the output struct. These names will be output verbatim, and are not eligible for renaming.