Add paragraph for saving new posts as drafts.

[henziger]

In the Django Forms tutorial chapter, the post.published_date member is set when we create the post.

This commit adds a short paragraph on removing that line which will allow us to create drafts that we later can publish with the publish button, making it more understandable to why we need to be able to list and publish drafts in the first hand.

[olasitarska]

Very useful, thank you @henziger!

Sorry for not merging this earlier -- this somehow completely disappeared from my radar.

[henziger]

This made my day! Thanks a lot :)