-> Readme added

README.md

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+# How it works
+
+1. Create new Class-Library for .NetFramework 4.7.2 or higher in Visual Studio.
+2. Open Nuget-Manager and install "PTConverter.Plugin"
+3. Add "PluginInfo.cs" and implement the interface "IPlugin".
+4. Create a XAML user control with the dimensions 450 x 800.
+5. Implement the "IPage" interface in your user control.
+
+   ```csharp
+   public string GetCategory() => "YourPluginPageName";
+   public UserControl GetPage() => new YourUserControl();
+   public string GetUnderCategory() => null; // null =  Single Page
+   public string GetUnderCategory() => "UnderPage"; // The main page has an underpage
+   public IPage Page => this;
+   ```
+
+6. Register your site in "PluginInfo.cs"
+
+   ```csharp
+   public IEnumerable<IPage> RegisterPages => new List<IPage>()
+   {
+       { new YourUserControl() }
+   };
+   ```
+
+7. Pack your library as DLL in the "plugins" folder of the application.
+
+8. Done.