Add Bash topic

[mgoulao]

Work in progress...

Discussion at #3

Task List:

• [x] Introduction
• [x] Commands in general (ls, cd, echo, man, etc)
• [x] Process Management and Conditional execution
• [x] Shortcuts
• [x] IO redirection
• [x] Permissions
• [x] Customization (.bashrc)

I might change the list

(closes #3)

[mgoulao]

I think we can improve a lot but for now, it's good enough.