Update README.md

Author: itn3000

README.md

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 [![NuGet version](https://badge.fury.io/nu/PooledStream.svg)](https://badge.fury.io/nu/PooledStream)
 
-[![Build status](https://ci.appveyor.com/api/projects/status/yhm2jto1ed0q0x0y/branch/master?svg=true)](https://ci.appveyor.com/project/itn3000/pooledstream-kviqp/branch/master)
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 this library aims to efficient MemoryStream for large data.
 
-**STILL UNDER DEVELOPMENT**
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 # Usage
 
 You can add reference as [NuGet package](https://www.nuget.org/packages/PooledStream/).