Native: add Native Custom Assets section (#2713)

* Native: add Custom Assets section

* wordsmithing

* Update native-implementation.md

Co-authored-by: bretg <bgorsline@gmail.com>

prebid/native-implementation.md

@@ -88,6 +88,7 @@ The Prebid.js AdUnit needs to defines a native mediatype object to tell bidders
 | ASSETCODE. aspect_ratios.min_height | optional | Part of the aspect_ratios object, bidders may pass this value through to the endpoint. Prebid.js does not enforce the responses and there’s no default. | 75 | integer |
 | ASSETCODE. aspect_ratios.ratio_width | optional | Part of the aspect_ratios object, bidders may pass this value through to the endpoint. Prebid.js does not enforce the responses and there’s no default. | 2 | integer |
 | ASSETCODE. aspect_ratios.ratio_height | optional | Part of the aspect_ratios object, bidders may pass this value through to the endpoint. Prebid.js does not enforce the responses and there’s no default. | 3 | integer |
+| ext.CUSTOMASSET | optional | Non-standard or bidder-specific assets can be supplied on the `ext` here. Attributes on custom assets are documented by the vendor. | | |
 
 
 **Table 3: Native Assets Recognized by Prebid.js**
@@ -101,6 +102,38 @@ Specific bidders may not support all of the fields listed below or may return di
 
 {% include dev-docs/native-image-asset-sizes.md %}
 
+### 3.2 Custom native assets
+
+In order to fit special bidder requirements, Prebid.js supports defining assets beyond the standard set. Simply define custom attributes in mediaTypes.native.ext, and they can be retrieved at render time. Other than being under the `ext` object, custom assets are declared in the same way as the standard ones.
+
+{: .alert.alert-success :}
+Note: The `native-render.js::renderNativeAd()` function must be called with `requestAllAssets: true`.
+
+Bid adapters will declare which custom assets they support in their documentation. It is recommended to prefix the asset name with the bidderCode to avoid collision issues.
+
+```javascript
+mediaTypes {
+  native: {
+    body: {
+      required: true
+    },
+    ext: {
+      bidderA_specialtracking: {    // custom asset
+        required: false
+      }
+  }
+}
+```
+
+In the native template, simply access the custom value with the normal Prebid ##macro## format assuming `hb_native_` as the prefix. For example:
+
+```
+    <div id="mytemplate">
+    ... render a lovely creative ...
+    ... refer to ##hb_native_bidderA_specialtracking## when appropriate ...
+    </div>
+```
+
 ## 4. Implementing the Native Template
 
 - If you want to manage your creative within the ad server (e.g. Google Ad Manager), follow the instructions for [AdServer-Defined Creative](#4-implementing-adserver-defined-creative).