Add OpenSky Network and remove Salesfly PDF (#1796)

README.md

@@ -378,7 +378,6 @@ API | Description | Auth | HTTPS | CORS |
 | [Pocket](https://getpocket.com/developer/) | Bookmarking service | `OAuth` | Yes | Unknown |
 | [PrexView](https://prexview.com) | Data from XML or JSON to PDF, HTML or Image | `apiKey` | Yes | Unknown |
 | [Restpack](https://restpack.io/) | Provides screenshot, HTML to PDF and content extraction APIs | `apiKey` | Yes | Unknown |
-| [Salesfly PDF](https://salesflypdf.com) | HTML/URL to PDF | `apiKey` | Yes | Yes |
 | [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 |
 | [WakaTime](https://wakatime.com/developers) | Automated time tracking leaderboards for programmers | No | Yes | Unknown |
@@ -1018,6 +1017,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Metro Lisboa](http://app.metrolisboa.pt/status/getLinhas.php) | Delays in subway lines | No | No | No |
 | [Navitia](https://api.navitia.io/) | The open API for building cool stuff with transport data | `apiKey` | Yes | Unknown |
 | [Open Charge Map](https://openchargemap.org/site/develop/api) | Global public registry of electric vehicle charging locations | `apiKey` | Yes | Yes |
+| [OpenSky Network](https://opensky-network.org/apidoc/index.html) | Free real-time ADS-B aviation data | No | Yes | Unknown |
 | [REFUGE Restrooms](https://www.refugerestrooms.org/api/docs/#!/restrooms) | Provides safe restroom access for transgender, intersex and gender nonconforming individuals | No | Yes | Unknown |
 | [Schiphol Airport](https://developer.schiphol.nl/) | Schiphol | `apiKey` | Yes | Unknown |
 | [TransitLand](https://transit.land/documentation/datastore/api-endpoints.html) | Transit Aggregation | No | Yes | Unknown |