Add README.md for System.Drawing.Primitives (#78123)

* Add README.md for System.Drawing.Primitives

* Update README.md

Author: ViktorHofer

src/libraries/System.Drawing.Primitives/README.md

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+# System.Drawing.Primitives
+This assembly exposes fundamental drawing APIs such as [`Point`](https://learn.microsoft.com/en-us/dotnet/api/system.drawing.point) and [`Size`](https://learn.microsoft.com/en-us/dotnet/api/system.drawing.size) which don't require access to native GDI+ libraries.
+
+Note that in contrast to `System.Drawing.Common`, `System.Drawing.Primitives` supports not just Windows, but all officially supported platforms.
+
+Documentation can be found at https://learn.microsoft.com/en-us/dotnet/api/system.drawing.
+
+## Contribution Bar
+- [x] [We consider new features, new APIs and performance changes](../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 that accept refactoring changes due to new language features](../README.md#secondary-bars)
+
+See the [Help Wanted](https://github.com/dotnet/runtime/issues?q=is%3Aissue+is%3Aopen+label%3Aarea-System.Drawing+label%3A%22help+wanted%22) issues.
+
+## Deployment
+`System.Drawing.Primitives` is part of the `Microsoft.NETCore.App` shared framework.