Update README.md

Author: winscripter

README.md

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 # Slnx
 SLNX is a fast parser and writer for the (currently) new in-preview Visual Studio XML Solution format with a .slnx extension, introduced in Visual Studio 2022 17.10 Preview 3.
 
+First came out on April 23, 2024 - days after SLNX file format was introduced.
+
 Available on NuGet: https://nuget.org/packages/Slnx
 
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