add a section about adding a .gitignore file to the tutorial

[pts]

The djangogirls tutorial asks the participant to add a .gitignore file (here: https://github.com/DjangoGirls/tutorial/blob/master/en/deploy/README.md) with the following contents:

*.pyc
*~
__pycache__
myvenv
db.sqlite3
/static
.DS_Store

The .DS_Store entry is relevant for this tutorial as well. Yesterday it happend on a Mac that macOS has created a .DS_Store file of 6100 bytes, and it kept updating it before each commit, polluting the repository each time.

The *~ entry is also relevant for this tuturial, to prevent committing backup files created by some text editor.

My proposal: add a section about adding a .gitignore file to the tutorial.