docs: announce venv creation before installing packages (#3729)

Author: nedbat

Makefile

@@ -64,6 +64,7 @@ venv:
 		echo "venv already exists."; \
 		echo "To recreate it, remove it first with \`make clean-venv'."; \
 	else \
+		echo "Creating venv in $(VENVDIR)"; \
 		$(PYTHON) -m venv $(VENVDIR); \
 		$(VENVDIR)/bin/python3 -m pip install -U pip wheel; \
 		$(VENVDIR)/bin/python3 -m pip install -r requirements.txt; \