Updated Extensibility (markdown)

Extensibility.md

@@ -1,6 +1,10 @@
-
 TODO: create a Visual Studio template for writing Library Installer extensions
 
+It's easy to write your own providers for the Library Installer.
+
+# Basic provider
+A basic provider allows libraries to be installed and removed. This is all you need to provide basic functionality for your provider.
+
 ## Step 1 
 Make sure the Visual Studio SDK is installed and then create a new extensibility project by creating a _VSIX Project_.
 
@@ -8,7 +12,16 @@ Make sure the Visual Studio SDK is installed and then create a new extensibility
 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.
+Implement the **IProviderFactory** and **IProvider** interfaces from the NuGet package and 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**.
+Mark the VSIX as a MEF export by opening the .vsixmanifest file and add the project as an MEF Component under **Assets**.
+
+# Advanced provider
+Contains the same steps as in the _Basic provider_. The advanced provider will give Intellisense in `library.json`, update checks and other features.
+
+## Step 1
+Implement **ILibraryCatalog** and the other interfaces used by it.
+
+## Step 2
+Return an instance of the **ILibraryCatalog** from the **IProvider.GetCatalog** method.