[en] Adding Operating System Book
Merge pull request #821 from esparta/master

[en] Adding Operating System Book
Update free-programming-books-zh.md
Merge pull request #822 from yangboz/patch-1

Update free-programming-books-zh.md
Added Speaking JavaScript online book from Dr. Axel Rauschmayer
Merge pull request #823 from thebouv/master

Added Speaking JavaScript online book from Dr. Axel Rauschmayer
[es] Adding a security book
Merge pull request #824 from esparta/master

[en] Adding a security book