script improvements and documentation

[robinwyss]

The PR contains the following changes

• Added the -d flag to the the start script, this way source maps are generated
• Copy .gitignore along with the rest of the files instead of downloading it from github, this also makes the script independent of the repo. Was there a specific reason to download the file github?
• Added some comments in the script to make it easier to read and understand

[Kornil]

Hi @robinwyss !
Thanks for your patience, as I was travelling in the last period your PR kinda slipped my mind.

Now on the code:
comments are an awesome first iteration! Really like the initiative and I would merge it if there wasn't the second part, the .gitignore copy instead of fetch.

.gitignore is not copied over in some OSes when you install the project globally with npm install -g simple-react-app so I could not copy it over like all the other files.
I could not replicate the bug myself on my OS (linux manjaro) but on a virtual mac the bug was there, everything is documented here #12

If you can remove the.gitignore change I will merge the PR. If you want to go the extra mile you can document why the .gitignore works this way ;)

[robinwyss]

@Kornil thanks for the feedback
I reverted the change to copy the .gitignore locally and added a comment about it in the script.

[Kornil]

Great job, merging!