Add libgen API (public-apis#834)

Author: s-nirali

README.md

@@ -115,6 +115,7 @@ API | Description | Auth | HTTPS | CORS |
 | [British National Bibliography](http://bnb.data.bl.uk/) | Books | No | No | Unknown |
 | [Goodreads](https://www.goodreads.com/api) | Books | `apiKey` | Yes | Unknown |
 | [Google Books](https://developers.google.com/books/) | Books | `OAuth` | Yes | Unknown |
+| [LibGen](http://garbage.world/posts/libgen/) | Library Genesis search engine | No | No | Unknown |
 | [Open Library](https://openlibrary.org/developers/api) | Books, book covers and related data | No | Yes | Unknown |
 | [Penguin Publishing](http://www.penguinrandomhouse.biz/webservices/rest/) | Books, book covers and related data | No | Yes | Unknown |