update readme (#500)

<!-- This is an auto-generated comment: release notes by OSS CodeRabbit
-->
### Summary by CodeRabbit

---
- **Documentation**: Updated README to reflect the project's transition
from "AI-based PR reviewer and summarizer" to "CodeRabbit Pro".
- **New Feature**: Introduced a new section promoting CodeRabbit Pro,
with added badges for Discord, license, and last commit information.
- **Chore**: Removed the section about CodeRabbit Pro from README, as it
is now available on coderabbit.ai.
- **Notice**: Added a note indicating that the old version is now in
maintenance mode and recommending users to install the Pro version.
---
<!-- end of auto-generated comment: release notes by OSS CodeRabbit -->

README.md

@@ -1,6 +1,12 @@
-# AI-based PR reviewer and summarizer
+# CodeRabbit Pro
+
+This is an old version of [CodeRabbit](http://coderabbit.ai) and is now in the maintenance mode.
+We recommend installing the Pro version from [CodeRabbit](http://coderabbit.ai). The Pro version is a total redesign and offers significantly better reviews that learn from your usage and improve over time. CodeRabbit Pro is free for open source projects. 
 
 [![Discord](https://img.shields.io/badge/Join%20us%20on-Discord-blue?logo=discord&style=flat-square)](https://discord.gg/GsXnASn26c)
+
+# AI-based PR reviewer and summarizer
+
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 [![GitHub](https://img.shields.io/github/last-commit/coderabbitai/ai-pr-reviewer/main?style=flat-square)](https://github.com/coderabbitai/ai-pr-reviewer/commits/main)
 
@@ -52,13 +58,6 @@ FAQs, you can refer to the sections below.
 - [Contribute](#contribute)
 - [FAQs](#faqs)
 
-## CodeRabbit Pro
-
-The professional version of `openai-pr-reviewer` project is now available at
-[coderabbit.ai](http://coderabbit.ai). Building upon our open source foundation,
-CodeRabbit Pro offers premium features including enhanced context and superior
-noise reduction, dedicated support, and our ongoing commitment to improve code
-reviews. Moreover, CodeRabbit Pro is free for open source projects.
 
 ## Install instructions