[Webpack Performance Guide] Add notes about webpack 4

src/content/en/fundamentals/performance/webpack/decrease-frontend-size.md

@@ -2,7 +2,7 @@ project_path: /web/fundamentals/_project.yaml
 book_path: /web/fundamentals/_book.yaml
 description: How to use webpack to make your app as small as possible
 
-{# wf_updated_on: 2017-12-18 #}
+{# wf_updated_on: 2018-02-24 #}
 {# wf_published_on: 2017-12-18 #}
 {# wf_blink_components: N/A #}
 
@@ -13,6 +13,10 @@ description: How to use webpack to make your app as small as possible
 One of the first things to do when you’re optimizing an application is to make it as small as
 possible. Here’s how to do this with webpack.
 
+Note: This page covers optimization strategies for webpack 3. A few parts
+of it aren’t necessary or work slightly differently with webpack 4. We’re waiting
+for the webpack ecosystem to stabilize, and then we’ll update this guide.
+
 ## Enable minification
 
 Minification is when you compress the code by removing extra spaces, shortening variable names and

src/content/en/fundamentals/performance/webpack/use-long-term-caching.md

@@ -2,7 +2,7 @@ project_path: /web/fundamentals/_project.yaml
 book_path: /web/fundamentals/_book.yaml
 description: How webpack helps with asset caching
 
-{# wf_updated_on: 2018-02-08 #}
+{# wf_updated_on: 2018-02-24 #}
 {# wf_published_on: 2018-02-08 #}
 {# wf_blink_components: N/A #}
 
@@ -14,6 +14,10 @@ The next thing (after [optimizing the app size](./decrease-frontend-size)) that
 improves the app loading time is caching. Use it to keep parts of the app on the
 client and avoid re-downloading them every time.
 
+Note: This page covers optimization strategies for webpack 3. A few parts
+of it aren’t necessary or work slightly differently with webpack 4. We’re waiting
+for the webpack ecosystem to stabilize, and then we’ll update this guide.
+
 ## Use bundle versioning and cache headers
 
 The common approach of doing caching is to: