[Docs] Added Content Extensions (#395)

Docs/MonoGame.Extended/Content.md

@@ -0,0 +1,64 @@
+# ContentManager extensions
+
+## ContentManager.OpenStream
+
+`System.IO.Stream ContentManager.OpenStream(string filename)`
+
+**OpenStream** allows easy access to `TitleContainer.OpenStream` so you can use the `Game.Content` object to load compiled resources *and* included resources.
+
+```csharp
+// my-file.txt is in the RootDirectory
+var stream = Content.OpenStream("my-file.txt");
+// do something with file
+stream.Close();
+```
+
+## ContentManager.GetGraphicsDevice
+
+`GraphicsDevice ContentManager.GetGraphicsDevice()`
+
+**GetGraphicsDevice** returns the current `GraphicsDevice` from the services.
+
+```csharp
+var graphicsDevice = Content.GetGraphicsDevice();
+var width = graphicsDevice.DisplayMode.Width;
+```
+
+# ContentReader extensions
+
+The `ContentReader` extensions help when writing your own content pipeline readers.
+
+## ContentReader.GetGraphicsDevice
+
+`GraphicsDevice ContentManager.GetGraphicsDevice()`
+
+**GetGraphicsDevice** returns the current `GraphicsDevice` to help when loading content for the current display.
+
+```csharp
+public class MyTypeReader : ContentTypeReader<MyType> {
+    protected override MyType Read(ContentReader reader, MyType existingInstance)
+    {
+        var graphicsDevice = reader.GetGraphicsDevice();
+        // ...
+    }
+}
+```
+
+## ContentReader.GetRelativeAssetName
+
+`string ContentReader.GetRelativeAssetName(string relativeName)`
+
+**GetRelativeAssetName** helps when your content type loads a different type, and you want to know the name to give `ContentManager.Load`.
+
+
+```csharp
+public class MyTypeReader : ContentTypeReader<MyType> {
+    protected override MyType Read(ContentReader reader, MyType existingInstance)
+    {
+        var assetName = reader.GetRelativeAssetName(reader.ReadString());
+        var other = reader.ContentManager.Load<OtherType>(assetName);
+        // ...
+    }
+}
+```
+

mkdocs.yml

@@ -12,6 +12,7 @@ pages:
     - Core:
         - BitmapFonts: MonoGame.Extended/BitmapFonts.md
         - Camera2D: MonoGame.Extended/Camera2D.md
+        - Content Extensions: MonoGame.Extended/Content.md
         - Collections: MonoGame.Extended/Collections.md
         - Object Pooling: MonoGame.Extended/Object-Pooling.md
         - Screens: MonoGame.Extended/Screens.md