[mutator] Update mutator readme (#406)

* [mutator] Update mutator readme

* [mutator] Fix _id placement in readme

packages/@sanity/mutator/README.md

@@ -1,21 +1,31 @@
 ## @sanity/mutator
+
 [![Build Status](https://travis-ci.org/sanity-io/mutator.svg?branch=master)](https://travis-ci.org/sanity-io/mutator)
 
 ## Features
-- Sanity flavored jsonpath matching engine with flexible interface that plays well with React
-- An implementation of the mutation operations of Sanity that can be applied to vanilla javascript objects, or through a flexible interface: any weird document representation you may require
-- TODO: A model to track documents as they are mutated both locally and remotely through the real time query feature of Sanity
+
+* Sanity flavored jsonpath matching engine with flexible interface that plays
+  well with React
+* An implementation of the mutation operations of Sanity that can be applied to
+  vanilla javascript objects, or through a flexible interface: any weird
+  document representation you may require
+* TODO: A model to track documents as they are mutated both locally and remotely
+  through the real time query feature of Sanity
+* Note: If `patch.id` doesn't match `document._id`, the patch will be ignored
+  during `apply`.
 
 ## Usage
 
 ```javascript
 import {Patcher} from '@sanity/mutator'
 
 const document = {
+  _id: 'a1b2c3',
   a: {}
 }
 
 const patcher = new Patcher({
+  id: 'a1b2c3',
   set: {
     'a.b': 'My new value'
   }
@@ -24,6 +34,7 @@ const patcher = new Patcher({
 console.log(patcher.apply(document))
 
 => {
+  _id: 'a1b2c3',
   a: {
     b: 'My new value'
   }