ContentExchange adapter docs (#3146)

* add contentexchange adapter

* fix

dev-docs/bidders/contentexchange.md

@@ -0,0 +1,21 @@
+---
+layout: bidder
+title: ContentExchange
+description: Prebid Contentexchange Bidder Adapter
+biddercode: contentexchange
+usp_supported: true
+schain_supported: true
+media_types: banner, video, native
+gdpr_supported: true
+pbjs: true
+pbs: false
+pbs_app_supported: false
+---
+
+### Prebid.JS Bid Params
+
+{: .table .table-bordered .table-striped }
+| Name           | Scope    | Description                                              | Example    | Type      |
+|----------------|----------|----------------------------------------------------------|------------|-----------|
+| `placementId` | required | Placement Id will be generated on ContentExchange Platform. | `'1234'`        | `string` |
+| `adFormat` | required | `[banner, video, native]` | `'banner'`        | `string` |