Add 'Helpful Links' slide to end of slide deck

[csndra0504]

Why

These are some helpful resources that we should share with the class!

What

1. Make a new slide before slide 17 https://gdi-orlando.github.io/gdi-featured-git-github/#/17
2. Add the following text:

Helpful links:

Videos and cheat sheet and visual git cheat sheet (under the Reference Manual):
https://git-scm.com/doc

Github’s Learning Lab:
https://lab.github.com/

Online tutorial:
https://try.github.io/

GitHub On Demand Training:
https://services.github.com/on-demand/