Add comment clarifying that Browser Sync is only for local development

Browser Sync doesn't play well with Glitch, so now it won't run if your
prototype is on Glitch, even if you have `useBrowserSync: true` in your
config file. I think this shouldn't actually affect users, because using
Browser Sync with Glitch doesn't make much sense (it won't work, and
Glitch auto-relaods for you anyway), but this commit adds a comment
anyway to make it clear that Browser Sync is only used for development
environments (the config variable has always been ignored when NODE_ENV
is true).

app/config.js

@@ -25,7 +25,7 @@ module.exports = {
   // Force HTTP to redirect to HTTPS on production
   useHttps: 'true',
 
-  // Enable or disable Browser Sync
+  // Enable or disable Browser Sync (local development only)
   useBrowserSync: 'true'
 
 }