[Doc] Add documentation with example about position.

doc/Notes/Theme/Positioning/theme3-positioning.markdown

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+# Positioning Rofi on the monitor
+
+In the current theme format you set these properties on the `window`  widget.
+
+The first, position, determines where **rofi** is placed on the monitor, the
+second what point of the **rofi** window connects there. This sounds
+complicated, but it ain't.
+
+## position setting
+
+The position setting determines the place of the window on the monitor.
+
+The position setting supports the following values:
+
+ * north
+ * northeast
+ * northwest
+ * south
+ * southeast
+ * southwest
+ * east
+ * west
+ * center
+
+This is depicted in the diagram below:
+
+![positions](anchors.svg)
+
+
+## anchor setting
+
+The anchor sets what point of the **rofi** window is placed at the specified *position*.
+
+The *anchor* settings supports the same values as the *position* setting.
+
+If you want the middle of the **rofi** window to be always located at the center of the monitor set both *position* and
+*anchor* to `center`.
+
+If the **rofi** window resizes, its center will stay at the center.
+If you set the *anchor* to `north` the top of the **rofi** window is at the center of the monitor, and the window will grow
+down.
+
+If you set the *anchor* and *position* to `south`, **rofi** is located at the bottom center and the window grows up.
+
+> Note that if you set the *anchor* to `south`  and the *position* to `north` the **rofi** window will be placed above
+> the monitor and might not be visible.
+
+
+> In another blog post we will explain how the dynamic sizing behaviour of **rofi** can be tweaked or disabled.
+
+So the following theme setting will place the top of the **rofi** window in the center of the monitor:
+
+```css
+window {
+    position: center;
+    anchor: north;
+}
+```
+
+As depicted here, RED is the position (center of screen), GREEN is the position on **rofi** window (north):
+
+![positions](example-pos.png)
+
+> Quick hint, if you want to quickly test out changes to the theme, without editing the file, run **rofi** like:
+
+```bash
+rofi -show run -theme-str "window { position: center; anchor: north;}"
+```