[Doc] Prerender.io configuration docs (#5)

* Added configuration docs for the prerender.io support

* Typo fixes

Author: jcalcaben

README.md

@@ -15,15 +15,18 @@ The Magento 2 UPWARD connector is configured in the admin area under:
 
 **Stores > Configuration > General > Web > UPWARD PWA Configuration**.
 
-### UPWARD Config File
+### General configuration
+
+These are the configurations for the UPWARD process itself.
+#### UPWARD Config File
 
 This configuration is the location of the UPWARD configuration file for the UPWARD-PHP server.
 
 _Use the absolute path on the server for the value of this configuration._
 
 Example: `/app/node_modules/@magento/venia-concept/dist/upward.yml`
 
-### Front Name Allowlist
+#### Front Name Allowlist
 
 This configuration allows you to specify a line-separated list of routes to forward to the default Magento theme.
 
@@ -36,6 +39,48 @@ privacy-policy-cookie-restriction-mode
 
 With this example, when a visitor navigates to either `<Magento store URL>/contact` or `<Magento store URL>/privacy-policy-cookie-restriction-mode`, they will land on a page rendered by Magento instead of the storefront application.
 
+### Prerender.io Configuration
+
+[Prerender.io][] support in the upward-connector module allows your site to send prerendered static html to search bots.
+
+A middleware layer checks each request to see if it comes from a crawler and if allowed, sends it to the prerender service.
+These configuration entries let you configure which pages to send to Prerender.io to serve the static HTML versions of that page.
+If a page is not configured for prerendering, the request continues using the normal server routes.
+
+| Configuration                    | Description                                                                                    | Example                       |
+| -------------------------------- | ---------------------------------------------------------------------------------------------- | ----------------------------- |
+| Enable Prerender For Search Bots | This enables prerender functionality for this store view.                                      |                               |
+| Prerender URL                    | Url of the prerender service.                                                                  | https://service.prerender.io/ |
+| Prerender.io Token               | Token to use for the prenderer.io hosted service                                               |                               |
+| Crawler User Agents              | Line break separated list of keywords to detect the crawler in the user-agent request header   |                               |
+| Blocked List                     | Resources that will not be sent for prerendering. Use `*` as a wildcard character.             | `.js` `*/cart`                |
+| Allowed List                     | Explicitly allowed resources to be sent for prerendering. If empty, all resources are allowed. |                               |
+
+#### Testing prerendered pages
+
+To see how a crawler sees a prerendered page, set your browser's User Agent to `Googlebot` and visit your URL.
+You can also run this on the command line and change the sample URL to your storefront's URL:
+
+``` sh
+curl -A Googlebot https://www.example.com/ > page.html
+```
+
+To configure prerender locally for testing purposes, see https://docs.prerender.io/test-it/.
+
+#### Troubleshooting partial rendered pages
+
+There is no way to tell when a PWA page fully loads.
+For prerendering it is possible to force prerender to wait for a predefined timeout before setting the `window.prerenderReady` flag.
+
+Add the following to the runtime script:
+
+``` js
+window.prerenderReady = false;
+setTimeout(function () { window.prerenderReady = true }, 1000 * 15)
+```
+
+For more information, see https://docs.prerender.io/test-it/.
+
 ## Service Worker Note
 
 Avoid sharing the same hostname between your PWA Studio storefront and the Magento 2 admin backend.
@@ -44,3 +89,4 @@ If you cannot avoid sharing the hostname, access one service at a time or use a
 
 [upward-php]: https://github.com/magento/upward-php
 [magento cloud deployment]: http://pwastudio.io/tutorials/cloud-deploy/
+[prerender.io]: https://docs.prerender.io/