docs(readme): explain tgclients library integration in local and docker setup

resolves !15 (comment 592925)

README.md

@@ -37,12 +37,19 @@ SPDX-License-Identifier: CC0-1.0
     python src/main.py
     ```
 
+-----
+Attention: This setup assumes you have the tgclients library source code installed at `../tgclients/`; see the [development requirements](requirements.dev.txt).
+
+-----
+
 For your convenience, a Docker Compose setup is delivered with this repository that configures the service accordingly. Build and start the stack.
 
 ```sh
 docker-compose up --build
 ```
 
+This setup does not rely on a local installation of the tgclients library but instead installs the (supposedly) latest and tested compatible version of the library; see the [requirements](requirements.txt).
+
 ## Contributing
 
 Commit convention: