Update free-programming-books.md

added pdf about openVZ virtualization for gnu/linux
Merge pull request #1186 from oswaldo-O/master

Update free-programming-books.md
[ja] follow-up #1179
Merge pull request #1187 from luminousspice/ja

[ja] follow-up #1179
Add A practical git introduction
Merge pull request #1188 from dhamaniasad/patch-4

Add "A practical git introduction"