add README files for Microsoft.Win32.* (#78248)

* Microsoft.Win32.Primitives: add README

* Microsoft.Win32.Registry: add README

* Microsoft.Win32.Registry.AccessControl: add README

* Microsoft.Win32.SystemEvents: add README

src/libraries/Microsoft.Win32.Primitives/README.md

@@ -0,0 +1,15 @@
+# Microsoft.Win32.Primitives
+
+This assembly exposes the types shared among the Win32 APIs like `System.ComponentModel.Win32Exception`.
+
+## Contribution Bar
+
+- [x] [We only consider fixes to maintain or improve quality](../README.md#primary-bar)
+- [x] [We consider PRs that target this library for new source code analyzers](../README.md#secondary-bars)
+- [x] [We consider PRs with refactoring changes due to new language features](../README.md#secondary-bars)
+
+See the [Help Wanted](https://github.com/dotnet/runtime/issues?q=is%3Aopen+is%3Aissue+label%3Aarea-Microsoft.Win32+label%3A%22help+wanted%22) issues.
+
+## Deployment
+
+`Microsoft.Win32.Primitives` is provided as a [NuGet package](https://www.nuget.org/packages/Microsoft.Win32.Primitives/) and part of the `Microsoft.NetCore.App` shared framework.

src/libraries/Microsoft.Win32.Registry.AccessControl/README.md

@@ -0,0 +1,15 @@
+# Microsoft.Win32.Registry.AccessControl
+
+This assembly provides APIs for managing access and audit control lists for Microsoft.Win32.RegistryKey.
+
+## Contribution Bar
+
+- [x] [We only consider fixes to maintain or improve quality](../README.md#primary-bar)
+- [x] [We consider PRs that target this library for new source code analyzers](../README.md#secondary-bars)
+- [x] [We consider PRs with refactoring changes due to new language features](../README.md#secondary-bars)
+
+See the [Help Wanted](https://github.com/dotnet/runtime/issues?q=is%3Aopen+is%3Aissue+label%3Aarea-Microsoft.Win32+label%3A%22help+wanted%22) issues.
+
+## Deployment
+
+`Microsoft.Win32.Registry.AccessControl` is provided as a [NuGet package](https://www.nuget.org/packages/Microsoft.Win32.Registry.AccessControl/) and part of the `Microsoft.WindowsDesktop.App` shared framework.

src/libraries/Microsoft.Win32.Registry/README.md

@@ -0,0 +1,15 @@
+# Microsoft.Win32.Registry
+
+This assembly provides APIs for accessing and modifying the Windows Registry.
+
+## Contribution Bar
+
+- [x] [We only consider fixes to maintain or improve quality](../README.md#primary-bar)
+- [x] [We consider PRs that target this library for new source code analyzers](../README.md#secondary-bars)
+- [x] [We consider PRs with refactoring changes due to new language features](../README.md#secondary-bars)
+
+See the [Help Wanted](https://github.com/dotnet/runtime/issues?q=is%3Aopen+is%3Aissue+label%3Aarea-Microsoft.Win32+label%3A%22help+wanted%22) issues.
+
+## Deployment
+
+`Microsoft.Win32.Registry` is provided as a [NuGet package](https://www.nuget.org/packages/Microsoft.Win32.Registry/) and part of the `Microsoft.NetCore.App` shared framework.

src/libraries/Microsoft.Win32.SystemEvents/README.md

@@ -0,0 +1,15 @@
+# Microsoft.Win32.SystemEvents
+
+This assembly provides support for accessing Windows system event notifications.
+
+## Contribution Bar
+
+- [x] [We only consider fixes to maintain or improve quality](../README.md#primary-bar)
+- [x] [We consider PRs that target this library for new source code analyzers](../README.md#secondary-bars)
+- [x] [We consider PRs with refactoring changes due to new language features](../README.md#secondary-bars)
+
+See the [Help Wanted](https://github.com/dotnet/runtime/issues?q=is%3Aopen+is%3Aissue+label%3Aarea-Microsoft.Win32+label%3A%22help+wanted%22) issues.
+
+## Deployment
+
+`Microsoft.Win32.SystemEvents` is provided as a [NuGet package](https://www.nuget.org/packages/Microsoft.Win32.SystemEvents/) and part of the `Microsoft.WindowsDesktop.App` shared framework.