Clarify how to run python code with repl or file

[brunogirin]

I had a student who was following the python tutorials recently and got confused between running code from a file or from the repl. One way to fix this would be:

• beginning of lesson 1, explain that everything will be done in the repl and how to start it
• end of lesson, add a note saying that you can save everything to a .py file and run it there

[KarinRoe]

I would like to work on this issue

[KimberleyCook]

@KarinRoe yes please do :)

[KarinRoe]

Send a pull request :) #424