Update README (Chrome & Opera section)

The Chrome extension is stable and has been around for a few years, so
the ancient note about the extension being experimental can be removed.
Also be a bit more specific about who maintains the Chrome extension,
to make it easier for external contributors to find who is responsible
for the development and maintenance of the PDF.js Chrome extension.

README.md

@@ -37,16 +37,13 @@ PDF.js is built into version 19+ of Firefox, however one extension is still avai
 
 #### Chrome and Opera
 
-The Chromium extension is still somewhat experimental but it can be installed two
++ The official extension for Chrome can be installed from the [Chrome Web Store](https://chrome.google.com/webstore/detail/pdf-viewer/oemmndcbldboiebfnladdacbdfmadadm).
-ways:
+*This extension is maintained by [@Rob--W](https://github.com/Rob--W).*
-
++ Opera has also published an extension for their browser at the [Opera add-ons catalog](https://addons.opera.com/en/extensions/details/pdf-viewer/).
-+ [Unofficial Version](https://chrome.google.com/webstore/detail/pdf-viewer/oemmndcbldboiebfnladdacbdfmadadm) - *This extension is maintained by a PDF.js contributor.*
 + Build Your Own - Get the code as explained below and issue `node make chromium`. Then open
 Chrome, go to `Tools > Extension` and load the (unpackaged) extension from the
 directory `build/chromium`.
 
-The version of the extension for the Opera browser can be found at the [Opera add-ons catalog](https://addons.opera.com/en/extensions/details/pdf-viewer/).
-
 ## Getting the Code
 
 To get a local copy of the current code, clone it using git: