poetry: add add --dev example (#7497)

pages/common/poetry.md

@@ -11,6 +11,10 @@
 
 `poetry add {{dependency}}`
 
+- Install a development dependency and its subdependencies:
+
+`poetry add --dev {{dependency}}`
+
 - Interactively initialize the current directory as a new Poetry project:
 
 `poetry init`