docs(gatsby-plugin-offline): specify to not HTTP-cache sw.js (gatsbyj…

…s#10430)

Fixes gatsbyjs#9671

docs/docs/caching.md

@@ -24,6 +24,8 @@ JavaScript and CSS files _generated by webpack_ should also be cached forever. L
 
 The `cache-control` header should be `cache-control: public, max-age=31536000,immutable`
 
+The only exception to this is the file `/sw.js`, which needs to be revalidated upon each load to check if a new version of the site is available. This file is generated by `gatsby-plugin-offline` and other service worker plugins, in order to serve content offline. Its `cache-control` header should be `cache-control: public, max-age=0, must-revalidate`
+
 ## Setting up caching on different hosts
 
 How you setup your caching depends on how you host your site. We encourage people to create Gatsby plugins per host to automate the creation of caching headers.