Add FAQ entry on cross-browser support (#5360)

Co-authored-by: Mathias Bynens <mathias@qiwi.be>

README.md

@@ -296,6 +296,17 @@ Check out [contributing guide](https://github.com/puppeteer/puppeteer/blob/maste
 The Chrome DevTools team maintains the library, but we'd love your help and expertise on the project!
 See [Contributing](https://github.com/puppeteer/puppeteer/blob/master/CONTRIBUTING.md).
 
+#### Q: What is the status of cross-browser support?
+
+Historically, Puppeteer supported Firefox indirectly through puppeteer-firefox, which relied on a custom, patched version of Firefox. This approach was also known as “Juggler”.
+After discussions with Mozilla, we collectively concluded that relying on custom patches was infeasible.
+Since then, we have been collaborating with Mozilla on supporting Puppeteer on “stock” Firefox.
+From Puppeteer v2.1.0 onwards, as an experimental feature, you can specify [`puppeteer.launch({product: 'firefox'})`](https://github.com/puppeteer/puppeteer/blob/v2.1.0/docs/api.md#puppeteerlaunchoptions) to run your Puppeteer scripts in Firefox, without any additional custom patches.
+
+We will continue collaborating with other browser vendors to bring Puppeteer support to browsers such as Safari.
+This effort includes exploration of a standard for executing cross-browser commands (instead of relying on the non-standard DevTools Protocol used by Chrome).
+
+
 #### Q: What are Puppeteer’s goals and principles?
 
 The goals of the project are: