Update README.md

Author: neuecc

README.md

@@ -821,6 +821,7 @@ Since there are no changes to the API, binary format, or behavior, you can migra
 * The code generator (MSBuild Task, .NET Core Global/Local Tools) has been removed and replaced with Source Generator
 * Tool options are now available through `MasterMemoryGeneratorOptions`
 * The `-addImmutableConstructor` option has been completely removed; please use C#'s record or init keyword instead
+* The library is now only available through NuGet for Unity. Please use NuGetForUnity
 
 License
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