Update page 'REST API'

REST API.md

@@ -76,3 +76,19 @@ GET http://localhost:1730/api/tracks/?album=115542362
 ``` http
 GET http://localhost:1730/api/queue/
 ```
+
+## Remove item from Download Queue
+``` JSON
+POST http://localhost:1730/api/canceldownload/
+Content-Type: application/json
+
+{
+    "queueId": "ida33m5l8firh"
+}
+```
+
+or
+
+``` http
+GET http://localhost:1730/api/canceldownload/?queueId=ida33m5l8firh
+```