Add doc for FairTrade Bidder Adapter (#608)

Author: VaticSSP

dev-docs/bidders/fairtrade.md

@@ -0,0 +1,20 @@
+---
+layout: bidder
+title: FairTrade
+description: FairTrade Bidder Adapter
+top_nav_section: dev_docs
+nav_section: reference
+hide: true
+biddercode: fairtrade
+biddercode_longer_than_12: false
+prebid_1_0_supported : true
+---
+
+
+### bid params
+
+{: .table .table-bordered .table-striped }
+| Name       | Scope    | Description                                                                                                                                                                                                  | Example |
+| :---       | :----    | :----------                                                                                                                                                                                                  | :------ |
+| `uid`      | required | Represents the FairTrade bidder system Ad Slot ID associated with the respective div id from the site page.                                                                                            | `5`     |
+| `priceType`| optional | Can take the values `gross` or `net`, default value is `net`. Net represents the header bid price with the header bidder margin already extracted. Gross price does contain the FairTrade bidder margin within. | `gross` |