Update free-programming-books.md
added Twitter share button

I couldn't add a Facebook share button because I don't have an account but here is the link if someone else wants to add it:

https://developers.facebook.com/docs/plugins/like-button/
Merge pull request #459 from breakingcrypto/patch-1

added Twitter share button
Merge pull request #458 from murphyrandle/master

Update free-programming-books.md
Fix and close #449
Add 'Ruby on Rails' section and 'El maldito libro de los Descarrilados' book
merge assembly with "ensamblador"

both catergories are the same. I also put the title of Lisa Watts's book in spanish (as the translations version has)
Merge pull request #462 from mgaitan/patch-1

merge assembly with "ensamblador" (spanish list)
Add index for 'Ruby on Rails' section.
Merge pull request #461 from Chakaitos/master

Update free-programming-books-es.md - new resource
Fixed link to How to be a Programmer
Merge pull request #463 from sugarjig/patch-1

Fixed link to How to be a Programmer
Update free-programming-books-pl.md
Merge pull request #464 from zawieja/master

Update for 'pl' file : Book added: Pisanie OS