Merge pull request #834 from esparta/master

[es] Adding book on software enginieering
Added books in czech language
Merge pull request #835 from grogy/add-czech-books

Added books in czech language
Fixed label of PDF files by list for contribute
Merge pull request #836 from grogy/fix-convention-for-contribute

Fixed label of PDF files by list for contribute
Added czech books about mathematic
Merge pull request #837 from grogy/czech-mathematic

Added czech books about mathematic
fixes multiple issues (broken links to be deleted)

fixes #738
fixes #727
fixes #725
added nicholas c. zakas understanding ECMAScript 6
Merge pull request #839 from hiteshtr/master

added nicholas c. zakas understanding ECMAScript 6
Merge pull request #838 from borgified/fix_738

fixes multiple issues (broken links to be deleted)
Added books by Peter Kropff
Verbesserungen
Merge pull request #840 from merkuriy/patch-1

[de] - Added books and slight improvements for German list
[pt_BR] - Kanban e Scrum - Fixes #832
[pt-BR] - Fixes #832
[pt_BR] - Fixing Index
added an intro to databases class
Merge pull request #841 from jay754/master

added an intro to databases class
added think complexity pdf