docs(readme): mention required python version

README.md

@@ -24,6 +24,8 @@ Diagrams lets you draw the cloud system architecture **in Python code**. It was
 
 ## Getting Started
 
+It requires **Python 3.6** or higher, check your Python version first.
+
 It uses [Graphviz](https://www.graphviz.org/) to render the diagram, so you need to [install Graphviz](https://graphviz.gitlab.io/download/) to use **diagrams**. After installing graphviz (or already have it), install the **diagrams**.
 
 > macOS users can download the Graphviz via `brew install graphviz` if you're using [Homebrew](https://brew.sh).