create useful git cmds with references in tools directory in repo

[palmoreck]

Git is very important to teach to students.

Next references are good ones:

https://product.hubspot.com/blog/git-and-github-tutorial-for-beginners (playground with git) https://learngitbranching.js.org/ http://geoscripting-wur.github.io/RProjectManagement/

Create directory under:

https://github.com/ITAM-DS/Propedeutico/tree/master/herramientas

Some useful tips

https://drivendata.github.io/cookiecutter-data-science/

[palmoreck]

Close #15