update instructions for build dependencies

* typo in sphinx plantuml package name
* make python and system dependencies explicit

README.md

@@ -24,10 +24,13 @@ TBD
 
 ## Building
 
-building the ki-doku depends on python, sphinx,
-sphinx-contrib-plantuml und plantuml.
+Building the ki-doku depends on the following Python packages:
+* `sphinx`
+* `sphinxcontrib-plantuml`
 
-building/ using the ki-backend depends on python and flask.
+as well as the system dependency `plantuml`, which you can get e.g. with `apt install plantuml`
+
+Building/using the ki-backend depends on python and flask, see the repo's environment specification for details.
 
 <!-- ROADMAP -->
 ## Roadmap