Add note about ExternalGraphics in README

Author: AD\akopinga

README.md

@@ -54,7 +54,46 @@ Wellknown names that reference a symbol library, like `ttf://CustomFont#42` are
 
 Only one Graphic per PointSymbolizer is supported. Each Graphic can only have one Mark or one ExternalGraphic.
 
+#### ExternalGraphic
+
+External graphics can be used with an `OnlineResource` linking to a valid image url.
+
+```xml
+<se:ExternalGraphic>
+  <se:OnlineResource xlink:type="simple" xlink:href="assets/img/flag-nl.png"/>
+  <se:Format>image/png</se:Format>
+</se:ExternalGraphic>
+```
+
+External graphics can also be embedded as base64 string using `InlineContent` with `encoding="base64"`.
+
+```xml
+<se:ExternalGraphic>
+  <se:InlineContent encoding="base64">iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==</se:InlineContent>
+  <se:Format>image/jpeg</se:Format>
+</se:ExternalGraphic>
+```
+
+Inline content can also be SVG with `encoding="xml"`.
+
+```xml
+<se:ExternalGraphic>
+  <se:InlineContent encoding="xml">
+    <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" width="100" height="100">
+      <path d="M50,3l12,36h38l-30,22l11,36l-31-21l-31,21l11-36l-30-22h38z" fill="#0F0" stroke="#040" stroke-width="2"/>
+    </svg>
+  </se:InlineContent>
+  <se:Format>image/svg+xml</se:Format>
+</se:ExternalGraphic>
+```
+
+**Important notes for inline SVG:**
+
+- Do not include the `<?xml ... ?>` header in the inline content.
+- Make sure that the SVG has `width` and `height` attributes. Without it, (most?) browsers cannot properly load the SVG as an image.
+
 #### QGIS parametric SVG support (experimental)
+
 SLD's with parametric embedded SVG's exported by QGIS should be able to be used in SLDReader.
 
 Support for this functionality is quite hacky and experimental, but appears to work for simple examples.