Add docs for apstream adapter (#2141)

* Add docs for apstream adapter

* Describe possibility to disable DSU

Co-authored-by: Yevhenii Tykhostup <yevhenii.tykhostup@audienceproject.com>

dev-docs/bidders/apstream.md

@@ -0,0 +1,97 @@
+# Overview
+
+```
+Module Name: AP Stream Bidder Adapter
+Module Type: Bidder Adapter
+Maintainer: stream@audienceproject.com
+gdpr_supported: true
+tcf2_supported: true
+```
+
+# Description
+
+Module that connects to AP Stream source
+
+# Inherit from prebid.js
+```
+    var adUnits = [
+      {
+        code: '/19968336/header-bid-tag-1',
+        mediaTypes: { // mandatory and should be only one
+            banner: {
+                sizes: [[920,180], [920, 130]]
+            }
+        },
+        bids: [{
+            bidder: 'apstream',
+            params: {
+                publisherId: STREAM_PIBLISHER_ID // mandatory
+            }
+        }]
+      }
+    ];
+```
+
+# Explicit ad-unit code
+```
+    var website = null;
+    switch (location.hostname) { 
+      case "site1.com": 
+        website = "S1"; 
+        break;
+      case "site2.com":
+        website = "S2";
+        break;
+    }
+    
+    var adUnits = [
+      {
+        code: '/19968336/header-bid-tag-1',
+        mediaTypes: { // mandatory and should be only one
+            banner: {
+                sizes: [[920,180], [920, 130]]
+            }
+        },
+        bids: [{
+            bidder: 'apstream',
+            params: {
+                publisherId: STREAM_PIBLISHER_ID, // mandatory
+                code: website + '_Leaderboard'
+            }
+        }]
+      }
+    ];
+```
+
+# Explicit ad-unit ID
+```
+    var adUnits = [
+      {
+        code: '/19968336/header-bid-tag-1',
+        mediaTypes: { // mandatory and should be only one
+            banner: {
+                sizes: [[920,180], [920, 130]]
+            }
+        },
+        bids: [{
+            bidder: 'apstream',
+            params: {
+                publisherId: STREAM_PIBLISHER_ID, // mandatory
+                adunitId: 1234
+            }
+        }]
+      }
+    ];
+```
+
+# DSU
+
+To disable DSU use config option:
+
+```
+    pbjs.setConfig({
+        apstream: {
+            noDsu: true
+        }
+    });
+```