Use a orthographic projection matrix like this to get y=0 at the bottom. Note that this example also does adds an offset of half a pixel to properly account for sampling texels. You may wish to change the far and near plane to whatever.
var viewport = GraphicsDevice.Viewport;
var projectionMatrix = Matrix.CreateTranslation(-0.5f, -0.5f, 0) * Matrix.CreateOrthographicOffCenter(0, viewport.Width, 0, viewport.Height, 0, 1);
See https://msdn.microsoft.com/en-us/library/bb195659.aspx for details on Matrix.CreateOrthographicOffCenter.