Native window decorations: added READMEs

flatlaf-natives/README.md

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+FlatLaf Native Libraries
+========================
+
+- [Windows 10 Native Library](flatlaf-natives-windows)
+- [Natives using JNA](flatlaf-natives-jna) (for development only)

flatlaf-natives/flatlaf-natives-jna/README.md

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+FlatLaf Natives using JNA
+=========================
+
+This sub-project contains source code that uses
+[JNA](https://github.com/java-native-access/jna) to access native operating
+system API.
+
+**Note:** Code in this sub-project is **not used** in FlatLaf libraries. It was
+used to develop/test usage of some native operating system API in Java (with the
+help of JNA) and was then converted to C++.

flatlaf-natives/flatlaf-natives-windows/README.md

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+FlatLaf Windows 10 Native Library
+=================================
+
+This sub-project contains the source code for the FlatLaf Windows 10 native
+library (DLL).
+
+The native library can be built only on Windows and requires a C++ compiler.
+Tested only with Microsoft Visual C++ 2019 (comes with Visual Studio 2019).
+
+To be able to build FlatLaf on any platform, and without C++ compiler, the
+pre-built DLL is checked into Git at
+`flatlaf-core/src/main/resources/com/formdev/flatlaf/natives/flatlaf-windows-x86_64.dll`.
+
+This DLL was built on a GitHub server with the help of GitHub Actions. See:
+[Native Libraries](https://github.com/JFormDesigner/FlatLaf/actions/workflows/natives.yml)
+workflow. Then the produced Artifacts ZIP was downloaded and checked into Git.