chore(readme): add customization information

Author: UziTech

README.md

@@ -5,3 +5,13 @@ Displays the search matches in the minimap.
 ![Minimap Screenshot](https://github.com/atom-minimap/minimap-find-and-replace/blob/master/screenshot.png?raw=true)
 
 You will need the [Minimap package](https://github.com/atom-minimap/minimap) installed to use this package.
+
+### Customization
+
+The color can be customized using the following CSS rule in your user stylesheet:
+
+```css
+.minimap .search-result {
+  background: yellow;
+}
+```