Add arXiv (#1769)

README.md

@@ -829,6 +829,7 @@ API | Description | Auth | HTTPS | CORS |
 API | Description | Auth | HTTPS | CORS |
 |---|---|---|---|---|
 | [arcsecond.io](https://api.arcsecond.io/) | Multiple astronomy data sources | No | Yes | Unknown |
+| [arXiv](https://arxiv.org/help/api/user-manual) | Curated research-sharing platform: physics, mathematics, quantitative finance, and economics | No | Yes | Unknown |
 | [CORE](https://core.ac.uk/services#api) | Access the world's Open Access research papers | `apiKey` | Yes | Unknown |
 | [GBIF](https://www.gbif.org/developer/summary) | Global Biodiversity Information Facility | No | Yes | Yes |
 | [hubblesite](https://hubblesite.org/api/documentation/) | Space Telescope News Releases | No | Yes | Unknown |