some stylish changes and conventions

Hi José,

Here you are your first pull request.

I thought it may be interesting to:
- name the files as .R for convention (instead of .txt) and also for your text editor to help you highligthing the code properly (eg: I use sublime text, wich does for R out of the box)
- add a comment in the first 'more info' line
- avoid redundant comments: just removed the "start function" and "end function" comments since that is already well stated by the code itsef. In my opinion, is better if the code talks for itsef and just comment when is not enough to understand. I know you write this also with a didactic intention (from your blog) so all those comments make sense, though.
- added some indentation. It helps to understand the code. To you, and even more to other people cloning your repo.

This are just suggestions, do what you like. Kind of a final practice for the course. :-)

Regards,

Javier