Docu: improve icons documentation

doc/Icons.md

@@ -1,3 +1,19 @@
+#### Unicode Icons
+Unicode defines many icons which can be used either directly or via their HTML codes, e.g.:
+```html
+🎂 🎂 🎂
+```
+
+A drawback of Unicode icons is, that the display will differ from system to system as they depend on the available Fonts.
+
+#### Self defined icons
+You may upload images to your repository and use them via a relative image link:
+```html
+<img src='img/foobar.svg'>
+```
+
+This will include the image from your repository (when uploaded to your 'img' directory).
+
 #### Font Awesome Icons
 Font Awesome 4 is included in OpenStreetBrowser, therefore you can use, e.g.:
 ```html
@@ -20,7 +36,7 @@ This will include the park-15.svg icon. Mapbox Maki icons come in two sizes: 11
 <img src="maki:park?size=11">
 ```
 
-You can pass SVG options to the icon to modify its look. Note that every icon is a path:
+You can pass URL options to the icon to modify its look. Note that every icon is a [SVG path](https://developer.mozilla.org/en-US/docs/Web/SVG/Tutorial/Paths) and all [style options](https://developer.mozilla.org/de/docs/Web/SVG/Tutorial/Fills_and_Strokes) are available:
 ```html
 <img src="maki:park?size=11&amp;fill=red&amp;stroke=black&amp;stroke-width=0.5">
 ```