Add ClickUp (#2449)

README.md

@@ -523,6 +523,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Api2Convert](https://www.api2convert.com/) | Online File Conversion API | `apiKey` | Yes | Unknown |
 | [apilayer pdflayer](https://pdflayer.com) | HTML/URL to PDF | `apiKey` | Yes | Unknown |
 | [Asana](https://developers.asana.com/docs) | Programmatic access to all data in your asana system | `apiKey` | Yes | Yes |
+| [ClickUp](https://clickup.com/api) | ClickUp is a robust, cloud-based project management tool for boosting productivity | `OAuth` | Yes | Unknown |
 | [Clockify](https://clockify.me/developers-api ) | Clockify's REST-based API can be used to push/pull data to/from it & integrate it with other systems | `apiKey` | Yes | Unknown |
 | [Cloudmersive Document and Data Conversion](https://cloudmersive.com/convert-api) | HTML/URL to PDF/PNG, Office documents to PDF, image conversion | `apiKey` | Yes | Yes |
 | [Code::Stats](https://codestats.net/api-docs) | Automatic time tracking for programmers | `apiKey` | Yes | No |