Added breaking changes in README

README.md

@@ -16,8 +16,9 @@ This crate works with Cargo and can be found on
 
 ```toml
 [dependencies]
-bson = "0.7"
+bson = "0.8"
 ```
+
 ## Usage
 Link the library in _main.rs_:
 
@@ -68,3 +69,29 @@ let person_document = mongoCollection.find_one(Some(doc! { "_id" => "12345" }),
 // Deserialize the document into a Person instance
 let person = bson::from_bson(bson::Bson::Document(person_document))?
 ```
+
+## Breaking Changes
+
+In the BSON specification, _unsigned integer types_ are unsupported; for example, `u32`. In the older version of this crate (< `v0.8.0`), if you uses `serde` to serialize _unsigned integer types_ into BSON, it will store them with `Bson::FloatingPoint` type. From `v0.8.0`, we removed this behavior and simply returned an error when you want to serialize _unsigned integer types_ to BSON. [#72](https://github.com/zonyitoo/bson-rs/pull/72)
+
+For backward compatibility, we've provided a mod `bson::compat::u2f` to explicitly serialize _unsigned integer types_ into BSON's floating point value as follows:
+
+```rust
+#[test]
+fn test_compat_u2f() {
+    #[derive(Serialize, Deserialize, Eq, PartialEq, Debug)]
+    struct Foo {
+        #[serde(with = "bson::compat::u2f")]
+        x: u32
+    }
+
+    let foo = Foo { x: 20 };
+    let b = bson::to_bson(&foo).unwrap();
+    assert_eq!(b, Bson::Document(doc! { "x" => (Bson::FloatingPoint(20.0)) }));
+
+    let de_foo = bson::from_bson::<Foo>(b).unwrap();
+    assert_eq!(de_foo, foo);
+}
+```
+
+In this example, we added an attribute `#[serde(with = "bson::compat::u2f")]` on field `x`, which will tell `serde` to use the `bson::compat::u2f::serialize` and `bson::compat::u2f::deserialize` methods to process this field.