Merge pull request #846 from esparta/master

[en] Adding an operating systems book
delete magazines as per #356
Merge pull request #847 from borgified/delete_magazines

delete magazines as per #356
Add Bootstrap section with book resource.
Merge pull request #851 from levibostian/bootstrap

Add Bootstrap section with book resource.
Add Git -- the simple guide
Add Git - the simple guide
Merge branch 'master' of github.com:holmboe/free-programming-books into holmboe-master
Alphabetizing Git
Merge pull request #853 from ilyar/master

Add Git - the simple guide [ru]
[en] Removing duplicated link on Parallel Programming section
[en] Sorting Parallel programming section