Add Builder::with for updating messages

Allow creating a new message based on an existing one, overriding
specific fields while copying the remaining original fields.

README.md

@@ -95,6 +95,23 @@ fn build_proto(buf: &mut [u8]) -> roto::Result<&[u8]> {
 Builder methods consume `self` and return `Result<Self>`, enabling `?`-based chaining.
 `finish()` returns `Result<&'b mut [u8]>` — a slice of the portion of the buffer that was written.
 
+### Updating messages
+
+You can read a message, modify specific fields, and use `.with()` to copy the remaining fields from the original binary.
+
+```rust
+fn update_proto(data: &[u8], buf: &mut [u8]) -> roto::Result<&[u8]> {
+    let msg = Message::new(data)?;
+
+    let mut builder = MessageBuilder::builder(buf);
+    if msg.foo()? == "bar" {
+        builder = builder.foo("foosbar")?;
+    }
+
+    builder.with(&msg)?.finish()
+}
+```
+
 ### Repeated fields
 
 Repeated fields return a `RepeatedFieldIterator<'a>`. Each item yields `Result<(&[u8], WireType)>`.