docs(readme): explain significance of Chromium revision (#4811)

Author: MaxGabriel

README.md

@@ -340,7 +340,7 @@ npm install puppeteer-core@chrome-71
 
 #### Q: Which Chromium version does Puppeteer use?
 
-Look for `chromium_revision` in [package.json](https://github.com/GoogleChrome/puppeteer/blob/master/package.json).
+Look for `chromium_revision` in [package.json](https://github.com/GoogleChrome/puppeteer/blob/master/package.json). To find the corresponding Chromium commit and version number, search for the revision prefixed by an `r` in [OmahaProxy](https://omahaproxy.appspot.com/)'s "Find Releases" section.
 
 #### Q: What’s considered a “Navigation”?