Add smartsheet (#1777)

README.md

@@ -155,6 +155,7 @@ API | Description | Auth | HTTPS | CORS |
 | [MailboxValidator](https://www.mailboxvalidator.com/api-email-free) | Validate email address to improve deliverability | `apiKey` | Yes | Unknown |
 | [mailgun](https://www.mailgun.com/) | Email Service | `apiKey` | Yes | Unknown |
 | [markerapi](http://www.markerapi.com/) | Trademark Search | No | No | Unknown |
+| [Smartsheet](https://smartsheet-platform.github.io/api-docs/) | Allows you to programmatically access and Smartsheet data and account information | `OAuth` | Yes | No |
 | [SwiftKanban](https://www.digite.com/swiftkanban/) | Kanban software, Visualize Work, Increase Organizations Lead Time, Throughput & Productivity | `apiKey` | Yes | Unknown |
 | [Ticksel](https://ticksel.com) | Friendly website analytics made for humans | No | Yes | Unknown |
 | [Trello](https://developers.trello.com/) | Boards, lists and cards to help you organize and prioritize your projects | `OAuth` | Yes | Unknown |