add purchase and free download links

[bpmutter]

i was thinking it could be nice if there were 2 additional columns on the table:

1. column with link to purchase book (good to support the authors!)
2. column with link to free download (good to have access to info if you can't afford to purchase!)

happy to add these if the maintainer(s) approve. let me know and i'll get started.

[mhadidg]

I'm considering adding a hyperlink on the book title referring to the book's official publisher page. Yet, I don't think adding a free download link is a good idea. It's an explicit violation to Github terms of service.

Thank you for the suggestion and the offer to add the links yourself. Yet, I already have a script that would semi-automate the process.

[guillermoarb]

Is just that a technical book costs three times more than a regular book. Learning is expensive, it shouldn't be that way. But a rule is a rule regarding term of service.