Step in packaging guide to create `setup.py' might cause confusion

[layday]

The "Creating the package files" step instructs readers to create a setup.py, which remains empty, leading to build failures:

• Trouble following packaging libraries tutorial packaging-problems#453
• ValueError: not enough values to unpack (expected 1, got 0) packaging-problems#454
• Trouble following packaging libraries tutorial packaging-problems#455

The guide later mentions that, "If you include [setup.py], you must have a call to setup() in it", but this appears to be easily overlooked.

[Nines9S]

Was scratching my head for ages wondering what I was doing wrong. Looks like I skimmed over that line too, mostly my fault, but it's very easily missed for sure. Finally got it working thanks to you!

[webknjaz]

Would turning it into an IMPORTANT admonition help?