Added some content

Linear-Gradient-Brush.md

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-TODO
+The _LinearGradientBrush_ is one of the default brushes provided by CUE.NET.  
+It's used to draw a gradient as a "line" inside a rectangle.  
+
+You are able to define a start and an end-point (both are inter- or extrapolated if needed) and the gradient which should be drawn on the virtual line between this points by either providing them as constructor-parameters or using the _StartPoint_-, _EndPoint_- and _Gradient_-Properties.  
+If you don't set an gradient the brush will return transparent for every key.  
+> The Start- and End-Point is given as the percentage from the top left corner.  
+> new PointF(0f, 0f) will be top left, new PointF(0,5f, 0,5f) in the center and new PointF(1f, 1f) in the bottom right.
+
+If you want to create a background on your keyboard looking like this
+
+<img src="https://upload.wikimedia.org/wikipedia/commons/2/27/Gradient.svg" width="128" height="128">
+
+you could use this code
+```C#
+GradientStop[] gradientStops =
+{
+    new GradientStop(0f, Color.Blue),
+    new GradientStop(1f, Color.Red)
+};
+LinearGradient blueToRedGradient = new LinearGradient(gradientStops); // Gradient from blue to red
+PointF startPoint = new PointF(0f, 1f); // Bottom left corner
+PointF endPoint = new PointF(1f, 0f); // Top right corner
+LinearGradientBrush linearBrush = new LinearGradientBrush(startPoint, endPoint, blueToRedGradient);
+keyboard.Brush = linearBrush;
+```