Update README: add badges and plugin migration for QuickLook.Common

Author: emako

QuickLook.Common/README.md

@@ -1,3 +1,24 @@
+[![GitHub license](https://img.shields.io/github/license/QL-Win/QuickLook.Common)](https://github.com/QL-Win/QuickLook/blob/master/QuickLook.Common/LICENSE) [![NuGet](https://img.shields.io/nuget/v/QuickLook.Common.svg)](https://nuget.org/packages/QuickLook.Common)
+
 # QuickLook.Common
 
 This repository holds the common library of QuickLook. The library is shared among all QuickLook projects.
+
+Repository: https://github.com/QL-Win/QuickLook
+
+## Plugin development update
+
+QuickLook.Common no longer requires submodule-based development for QuickLook plugins.
+
+If your plugin project previously used this repository as a git submodule, remove it with:
+
+```bash
+git submodule deinit -f QuickLook.Common
+git rm -f QuickLook.Common
+```
+
+Then switch to NuGet dependency:
+
+```xml
+<PackageReference Include="QuickLook.Common" Version="x.y.z" />
+```