Update README

Author: valeriechen

README.md

@@ -18,8 +18,6 @@ Copilot Arena is an open source AI coding assistant that provides paired autocom
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-Code coming soon!
-
 ## Read This To Get Started
 
 Step 1: Download the Copilot Arena extension from the [Visual Studio Code Store](https://marketplace.visualstudio.com/items?itemName=copilot-arena.copilot-arena). 
@@ -34,6 +32,8 @@ Step 4: Create a username by clicking the `Copilot Arena` icon on the sidebar; d
 
 You are now ready to get coding with Copilot Arena!
 
+**New!** We have recently open-sourced our backend code. Check out [this README](server/README.md) if you are interested in contributing prompts and new models. We welcome folks to get involved to improve Copilot Arena!
+
 ## Table of Contents
 
 - [In-line editing](#in-line-editing)