add UNICORN bid adapter docs (#1827)

* add UNICORN bid adapter documentation

* Update unicorn.md

Co-authored-by: Jean Stemp <38964447+jeanstemp@users.noreply.github.com>

dev-docs/bidders/unicorn.md

@@ -0,0 +1,18 @@
+---
+layout: bidder
+title: UNICORN
+description: Prebid UNICORN Bidder Adaptor
+hide: true
+media_types: banner
+biddercode: unicorn
+---
+
+### bid params
+
+{: .table .table-bordered .table-striped }
+| Name          | Scope    | Description                               | Example    | Type      |
+|---------------|----------|-------------------------------------------|------------|-----------|
+| `placementId` | optional | Your placement ID | `'rectangle-ad-1'` | `string`  |
+| `accountId` | required | Account ID for charge request (provided by UNICORN) | `12345`    | `integer` |
+| `bidfloorCpm` | optional | Floor CPM (JPY); defaults to 0.  | `0.2`    | `float` |
+| `bcat` | optional | Blocked IAB categories | `['IAB-1', 'IAB-2']`    | `[string]` |