first version of translation section 6.1
add new line at the end of file, as pointed by jnavila
Merge pull request #451 from gibas/06-git-tools

[pl] translation of section 6.1
inprovements in section 6.1, translation of section 6.2
Merge pull request #452 from gibas/06-git-tools

[pl] inprovements in section 6.1, translation of section 6.2
translation of section 6.3
Merge pull request #453 from gibas/06-git-tools

[pl] translation of section 6.3
Merge pull request #454 from harupong/master

[ja] Update chapter4