Update README regarding migration (#1302)

Author: Andrew-Chen-Wang

CHANGES/1302.misc

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+Migrating to redis-py README update :)

README.md

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 # aioredis
 
+---
+
+## 📢🚨 Aioredis is now in redis-py 4.2.0rc1+ 🚨🚨
+
+Aioredis is now in redis-py 4.2.0rc1+
+
+To install, just do `pip install redis>=4.2.0rc1`. The code is almost the exact same. You will just need to import like so:
+
+```python
+from redis import asyncio as aioredis
+```
+
+This way you don't have to change all your code, just the imports.
+
+https://github.com/redis/redis-py/releases/tag/v4.2.0rc1
+
+Now that aioredis is under Redis officially, I hope there will never be an unmaintained, asyncio Redis lib in the Python ecosystem again. I will be helping out maintenance at Redis-py for the foreseeable future just to get some of the asyncio stuff out of the way. There are also some bugs that didn't make it into the [PR](https://github.com/redis/redis-py/pull/1899) that I'll be slowly migrating over throughout the next few weeks -- so long as my exams don't kill me beforehand :)
+
+Thank you all so much for your commitment to this repository! Thank you so much to @abrookins @seandstewart @bmerry for all the commits and maintenance. And thank you to everyone here who has been adopting the new code base and squashing bugs. It's been an honor!
+
+Cheers,
+Andrew
+
+---
+
 asyncio (3156) Redis client library.
 
 The library is intended to provide simple and clear interface to Redis