wiki: Add screenshot to window effects

docs/wiki/Window-Effects.md

@@ -6,6 +6,8 @@ You can apply background effects to windows and layer-shell surfaces.
 These include blur, xray, saturation, and noise.
 They can be enabled in the `background-effect {}` section of [window](./Configuration:-Window-Rules.md#background-effect) or [layer](./Configuration:-Layer-Rules.md#background-effect) rules.
 
+![Screenshot with blur](./img/blur.png)
+
 The window needs to be semitransparent for you to see the background effect (otherwise it's fully covered by the opaque window).
 Focus ring and border can also cover the background effect, see [this FAQ entry](./FAQ.md#why-are-transparent-windows-tinted-why-is-the-borderfocus-ring-showing-up-through-semitransparent-windows) for how to change this.
 

docs/wiki/img/blur.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:44cbf3144d7add741762a8034779602f6a94b2b99a6d62774c9d75f3606b98b1
+size 1582702