Add dev doc for Appier bidder. (#1058)

* Add dev doc for Appier bidder.

* adding quotes around string data example

dev-docs/bidders/appier.md

@@ -0,0 +1,57 @@
+---
+layout: bidder
+title: Appier
+description: Prebid Appier Bidder Adaptor
+top_nav_section: dev_docs
+nav_section: reference
+biddercode: appier
+biddercode_longer_than_12: false
+hide: true
+prebid_1_0_supported : true
+media_types: banner
+gdpr_supported: true
+---
+
+### Table of Contents
+
+- [Bid Params](#appier-bid-params)
+- [Custom Settings](#appier-custom-settings)
+
+
+<a name="appier-bid-params" />
+
+#### Bid Params
+
+{: .table .table-bordered .table-striped }
+
+| Name      | Scope    | Description               | Example    | Type     |
+|-----------|----------|---------------------------|------------|----------|
+| `hzid`    | required | The zone ID from Appier.  | `"WhM5WIOp"` | `string` |
+
+
+<a name="appier-custom-settings" />
+
+#### Custom Settings (Optional)
+
+Set the "farm" to use region-specific server
+```
+pbjs.que.push(function() {
+  // use the bid server in Taiwan (country code: tw)
+  pbjs.setConfig({
+    appier: {
+      'farm': 'tw'
+    }
+  });
+});
+```
+
+Explicitly override the bid server used for bidding
+```
+pbjs.que.push(function() {
+  pbjs.setConfig({
+    appier: {
+      'server': '${HOST_NAME_OF_THE_SERVER}'
+    }
+  });
+});
+```