revisions to Deploy chapter

Based on @judy2k's feedback from Django Girls Amsterdam - his group had some problems with the end of the Deploy chapter, where they saw an error message when they first open the app after deploying - the error was mentioned but not very clearly, and caused some confusion.  I also made some English edits again to the beginning section :)
I added this because we weren't saying what git was at all, but I didn't want to go into too much detail in case it was unnecessarily confusing.  I think you're right, though, it needs a bit more explanation.  I added another bit here.