doc: Add steps to install uvloop in extension README

Author: arenekosreal

webext/README.md

@@ -38,6 +38,12 @@ couple of options:
   echo "#!$(readlink -f ./env/bin/python3)"
   ```
 
+Then, optionally, you can use [uvloop](https://github.com/MagicStack/uvloop)>=0.18 as the web extension's async event loop implementation.
+This is not required, we prefer using uvloop, while can still fallback to use the built-in asyncio if failed to import it.
+
+If you are sure to use it, simply install `uvloop` like installing `dbus-next` mentioned above.
+We will print a log to let you know if we are using uvloop.
+
 # Setup Instructions
 
 ## For Testing