Add usage example for the interval tree.

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

Author: jiangliu

src/interval_tree.rs

@@ -5,6 +5,36 @@
 //!
 //! It's not designed as a generic interval tree, but specialized for VMM resource management.
 //! In addition to the normal get/insert/delete/update operations, it also implements allocate/free.
+//!
+//! # Examples
+//! ```rust
+//! extern crate vm_allocator;
+//! use vm_allocator::{IntervalTree, Range, Constraint};
+//!
+//! // Create an interval tree and add availale resources into it.
+//! let mut tree = IntervalTree::<u64>::new();
+//! tree.insert(Range::new(0x100, 0x100), None);
+//! tree.insert(Range::new(0x200, 0x2ff), None);
+//!
+//! // Allocate a range with constraints.
+//! let mut constraint = Constraint::new(8);
+//! constraint.min = 0x211;
+//! constraint.max = 0x21f;
+//! constraint.align = 0x8;
+//!
+//! let key = tree.allocate(&constraint);
+//! assert_eq!(key, Some(Range::new(0x218, 0x21f)));
+//!
+//! // Associate data with the allocated range and mark the range as occupied.
+//! // Note: caller needs to protect from concurrent access between allocate() and the first call
+//! // to update() to mark range as occupied.
+//! let old = tree.update(&Range::new(0x218, 0x21f), 2);
+//! assert_eq!(old, None);
+//! let old = tree.update(&Range::new(0x218, 0x21f), 3);
+//! assert_eq!(old, Some(2));
+//! let val = tree.get(&Range::new(0x218, 0x21f));
+//! assert_eq!(val, Some(Some(&3)));
+//! ```
 
 use crate::{AllocPolicy, Constraint};
 use std::cmp::{max, min, Ordering};