Merge pull request #1551 from alcole/alcole-books-crossref

Added Crossref rest api under books

README.md

@@ -124,6 +124,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Bhagavad Gita](https://bhagavadgita.io/api) | Bhagavad Gita text | `OAuth` | Yes | Yes |
 | [Bible](https://bibleapi.co/) | RESTful Bible API with 7 versions, 4 languages and multiple features | `apiKey` | Yes | Unknown |
 | [British National Bibliography](http://bnb.data.bl.uk/) | Books | No | No | Unknown |
+| [Crossref Metadata Search](https://github.com/CrossRef/rest-api-doc) | Books & Articles Metadata | No | Yes | Unknown |
 | [Google Books](https://developers.google.com/books/) | Books | `OAuth` | Yes | Unknown |
 | [LibGen](https://garbage.world/posts/libgen/) | Library Genesis search engine | No | No | Unknown |
 | [New York Times Books](https://developer.nytimes.com/docs/books-product/1/overview) | Book reviews and The New York Times Best Sellers lists | `apiKey` | Yes | Unknown |