docs: document using custom SVG icons without a gulp build step (#794)

customize.md only described the src/icons -> gulp build-icons -> gulp
css icon-font pipeline, which is meant for icons bundled into this
package's own build/fork. It never mentioned that the icon option
already supports plain CSS classes or a function injecting inline SVG,
so users following the docs hit an unnecessary build/fork requirement
for something already possible.

Fixes #762

Author: bbrala

documentation/docs/customize.md

@@ -25,6 +25,30 @@ Font-Awesome icons used from [encharm/Font-Awesome-SVG-PNG](https://github.com/e
 
 Finally, you will need to re-build the CSS using [`sass`](http://sass-lang.com), otherwise you may see mismatchings between the icon references. Use the command ``gulp css`` to re-build the CSS in the dist directory. The new CSS files will contain the icons you added.
 
+### Using your own SVG icons without a build step
+
+The `gulp build-icons` pipeline above compiles the SVGs in `src/icons` into an icon font, which is meant for icons you want bundled into this package's own build. If you just want to use your own SVG icons in your project, you don't need to fork or build anything, since the [icon option](items#icon) already supports both of the following, plain CSS approaches:
+
+```javascript
+var items = {
+    firstCommand: {
+        name: "Paste",
+        // context-menu-icon-checkmark must be defined in your own CSS, e.g.
+        // .context-menu-icon-checkmark { background-image: url(checkmark.svg); }
+        icon: "checkmark"
+    },
+    secondCommand: {
+        name: "Copy",
+        // or inject an inline <svg> (or <img>) directly into the item and
+        // return a class name to mark it as done (see the icon option docs)
+        icon: function (opt, $itemElement) {
+            $itemElement.prepend('<svg class="context-menu-icon" ...>...</svg>');
+            return 'context-menu-icon-inline';
+        }
+    }
+}
+```
+
 ## Customize CSS
 
 You can use the _variables.scss to adjust variables on pretty much everything you want to change.