Update free-programming-books.md

Fixed typo.
added learn pandas to the list
Merge pull request #811 from bmcculley/adding_a_resource

added learn pandas to the list (python)
Apuntes de Algoritmos y Estructuras de Datos
Merge pull request #812 from alejolp/master

Apuntes de Algoritmos y Estructuras de Datos
Updated JavaScript list

Added Google JavaScript Style Guide. Also alphabetized the JavaScript list.
Updated PHP book

Added link to PHP security guide.
Updated PHP books
Added OS book: Operating System Concepts
Add ML book: AI, A Modern Approach
Merge pull request #816 from esparta/master

[es] Adding new Section & book: SQL
Merge pull request #815 from ScottL/master

Added 1 OS book, 1 AI book
Merge pull request #814 from virendrachandak/patch-2

Updated PHP list
Added MySQL book

Added MySQL book
Merge pull request #817 from virendrachandak/patch-2

Added MySQL book
Merge pull request #818 from sjmcallister/master

Update free-programming-books.md
Merge pull request #813 from virendrachandak/patch-1

Updated JavaScript list + Google Javascript
Adicionado Grunt

Adicionado artigo sobre Grunt
Merge pull request #820 from wbecher/master

Adicionado Grunt
[en] Adding Operating System Book