docs(README.md): include older version of poetry

Author: Aleksandar Ivanovski

docs/README.md

@@ -58,10 +58,18 @@ You can add it to your local project using one of these:
 pip install -U commitizen
 ```
 
+for Poetry >= 1.2.0:
+
 ```bash
 poetry add commitizen --group dev
 ```
 
+for Poetry < 1.2.0:
+
+```bash
+poetry add commitizen --dev
+```
+
 ### macOS
 
 On macOS, it can also be installed via [homebrew](https://formulae.brew.sh/formula/commitizen):