📝 (api) Add installation instructions

Author: simonwoerpel

docs/api.md

@@ -1,5 +1,11 @@
 `leakrfc` provides a simpel api powered by [FastAPI](https://fastapi.tiangolo.com/) for clients to retrieve file metadata and blobs. It therefore acts as a proxy between client and archive, so that the client doesn't need to know where the actual blobs live. The api can handle authorization via [JSON Web Tokens](https://jwt.io).
 
+## Installation
+
+The API feature needs some extra packages that are not installed by default. Install `leakrfc` with api dependencies:
+
+    pip install leakrfc[api]
+
 ## Start local api server
 
 This is for a quick testing setup: