Created Extensibility (markdown)

Extensibility.md

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+TODO: create a Visual Studio template for writing Library Installer extensions
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+## Step 1 
+Make sure the Visual Studio SDK is installed and then create a new extensibility project by creating a _VSIX Project_.
+
+## Step 2
+Install the **Microsoft.Web.LibraryInstaller.Contracts** NuGet package
+
+## Step 3
+Implement the **IProviderFactory** and **IProvider** interfaces from the NuGet package add an `[Export(IProviderFactory)]` attribute to the **IProviderFactory** class.
+
+## Step 4
+Mark the VSIX as a MEF export by opening the .vsixmanifest file and add the project as an MEF Component under **Assets**.