Add cache-control examples to caching docs page (gatsbyjs#3468)

docs/docs/caching.md

@@ -6,12 +6,24 @@ An important part of creating a very fast website is setting up proper HTTP cach
 
 Different types of resources are cached differently. Let's examine how the different types of files built to `public/` should be cached.
 
+## HTML
+
 HTML files should never be cached. When you rebuild your Gatsby site, you often update the contents of HTML files. Because of this, browsers should be instructed to check on every request if they need to download a newer version of the HTML file.
 
+The `cache-control` header should be `cache-control: public, max-age=0, must-revalidate`
+
+## Static files
+
 All files in `public/static/` should be cached forever. For files in this directory, Gatsby creates paths that are directly tied to the content of the file. Meaning that if the file content changes, then the file path changes also. These paths look weird e.g. `reactnext-gatsby-performance.001-a3e9d70183ff294e097c4319d0f8cff6-0b1ba.png` but since we know that we'll always get the same file when we request that path, we can cache it forever.
 
+The `cache-control` header should be `cache-control: public,max-age=31536000,immutable`
+
+## JavaScript
+
 Other files e.g. JavaScript files are _not_ (yet) cachable. Gatsby v1 is using webpack 1 which doesn't make it possible to produce paths tied directly to the content of the file. Gatsby v2 will ship with webpack 3 which [will make possible long-term caching of our JavaScript files](https://medium.com/webpack/predictable-long-term-caching-with-webpack-d3eee1d3fa31).
 
+The `cache-control` header should be `cache-control: public, max-age=0, must-revalidate`
+
 How you setup caching depends on how you're hosting your site. We encourage people to create Gatsby plugins which automate the creation of caching headers for Gatsby sites. The following plugins have been created:
 
 * [gatsby-plugin-netlify](/packages/gatsby-plugin-netlify/)