Add SLF (#2187)

README.md

@@ -642,6 +642,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Riot Games](https://developer.riotgames.com/) | League of Legends Game Information | `apiKey` | Yes | Unknown |
 | [RuneScape](https://runescape.wiki/w/Application_programming_interface) | RuneScape and OSRS RPGs information | No | Yes | No |
 | [Scryfall](https://scryfall.com/docs/api) | Magic: The Gathering database | No | Yes | Yes |
+| [SLF](https://github.com/slftool/slftool.github.io/blob/master/API.md) | German city, country, river, database | No | Yes | Yes |
 | [SpaceTradersAPI](https://spacetraders.io?rel=pub-apis) | A playable inter-galactic space trading MMOAPI | `OAuth` | Yes | Yes |
 | [Steam](https://steamapi.xpaw.me/) | Steam Web API documentation | `apiKey` | Yes | No |
 | [Steam](https://github.com/Revadike/InternalSteamWebAPI/wiki) | Internal Steam Web API documentation | No | Yes | No |