Improve readme - what even is this? (#2837)

* Improve readme - what even is this?

Propose words for #2836

* Update README.md

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Co-authored-by: Nick Craver <nrcraver@gmail.com>

Author: mgravell

README.md

@@ -1,6 +1,8 @@
 StackExchange.Redis
 ===================
 
+StackExchange.Redis is a .NET client for communicating with RESP servers such as [Redis](https://redis.io/), [Garnet](https://microsoft.github.io/garnet/), [Valkey](https://valkey.io/), [Azure Cache for Redis](https://azure.microsoft.com/products/cache), [AWS ElastiCache](https://aws.amazon.com/elasticache/), and a wide range of other Redis-like servers. We do not maintain a list of compatible servers, but if the server has a Redis-like API: it will *probably* work fine. If not: log an issue with details!
+
 For all documentation, [see here](https://stackexchange.github.io/StackExchange.Redis/).
 
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