Adds RenderPDF.io API (#339)

RenderPDF.io

README.md

@@ -645,6 +645,7 @@
 | [Pocket](https://getpocket.com/developer/) | Bookmarking service | `OAuth`| Yes | Unknown |
 | [Podio](https://developers.podio.com) | File sharing and productivity | `OAuth`| Yes | Unknown |
 | [PrexView](https://prexview.com) | Data from XML or JSON to PDF, HTML or Image | `apiKey` | Yes | Unknown |
+| [RenderPDF.io](https://renderpdf.io) | HTML to PDF crazy fast, 500 pdfs/month + CDN-ready | `apiKey` | Yes | Yes |
 | [Restpack](https://restpack.io/) | Provides screenshot, HTML to PDF and content extraction APIs | `apiKey` | Yes | Unknown |
 | [Todoist](https://developer.todoist.com) | Todo Lists | `OAuth`| Yes | Unknown |
 | [Vector Express v2.0](https://vector.express) | Free vector file converting API | No | Yes | No |