Update README.md

Author: MiloszKrajewski

README.md

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 **This library is no longer maintained and has been replaced by [K4os.Compression.LZ4](https://github.com/MiloszKrajewski/K4os.Compression.LZ4).**
 
 K4os.Compression.LZ4 is a port of much more recent version of lz4 for .NET Standard and handles both BLOCK and STREAM modes.
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+Old README.md can be found [here](doc/readme.md)