add imuIdSystem doc (#3159)

* add imuIdSystem doc

* cid is number

dev-docs/modules/userId.md

@@ -792,6 +792,46 @@ pbjs.setConfig({
 });
 {% endhighlight %}
 
+### IM-UID by Intimate Merger
+
+IM-UID, provided by [Intimate Merger](https://corp.intimatemerger.com/), is a universal identifier that designed for publishers, platforms and advertisers to perform segmentation and targeting even in environments where 3rd party cookies are not available. IM-UID is currently only available in Japan.
+
+Add it to your Prebid.js package with:
+
+{: .alert.alert-info :}
+gulp build --modules=imuIdSystem
+
+#### IM-UID Registration
+
+Please visit [https://lp.intimatemerger.com/im-uid](https://lp.intimatemerger.com/im-uid) and request your Customer ID to get started.
+
+The Intimate Merger privacy policy is at https://corp.intimatemerger.com/privacypolicy/
+
+#### IM-UID Configuration
+
+{: .table .table-bordered .table-striped }
+| Param under userSync.userIds[] | Scope | Type | Description | Example |
+| --- | --- | --- | --- | --- |
+| name | Required | String | The name of this module. | `"imuid"` |
+| params | Required | Object | Details of module params. | |
+| params.cid | Required | Number | This is the Customer ID value obtained via Intimate Merger. | `5126` |
+| params.url | Optional | String | Use this to change the default endpoint URL. | `"https://example.com/some/api"` |
+
+#### IM-UID Example
+
+{% highlight javascript %}
+pbjs.setConfig({
+    userSync: {
+        userIds: [{
+            name: "imuid",
+            params: {
+                cid: 5126 // Set your Intimate Merger Customer ID here for production
+            }
+        }]
+    }
+});
+{% endhighlight %}
+
 ### Intent IQ ID
 
 Intent IQ’s universal ID with its unparalleled coverage of over 80% of ad inventory, protects publishers’ ability to rely on advertising as their main revenue source while preserving user privacy in a third party cookieless world.