Initial documentation with example of Output mapping feature

docs/map.md

@@ -1,5 +1,38 @@
 # Output mapping feature
 
-You can pass a function as the `map` option to have full control over your snapshot output.
+You can pass a function as the `map` option to have full control over your snapshot output. The function will traverse each node of your _wrapper_ tree recursively.
 
-* Documentation coming soon *
+## Example
+
+Use `map` to remove `containerInfo` prop from `Portal` components to avoid brittle snapshots.
+
+```javascript
+const removePortalContainerInfo = json => {
+  if (json.type === "Portal") {
+    return {
+      ...json,
+      props: {
+        ...json.props,
+        containerInfo: undefined
+      }
+    };
+  }
+  return json;
+};
+
+// Mount a component that might render a portal
+const wrapper = mount(MyComponent);
+
+// Then in your matcher
+
+expect(
+  toJson(wrapper, {
+    mode: "deep",
+    map: wrapper.exists("Portal") && removePortalContainerInfo
+  })
+).toMatchSpecificSnapshot(snapshotFilename);
+```
+
+Now these snapshots are solid. 
+
+This example was extracted from a project using Material UI and the snapshot is actually a StoryShot. Before using the `map` function `containerInfo` was randomly set with body and spans apparently coming from Storybook markup.