Zhtw 7: Branches Aren't Just For Birds

Added zhtw translation for chapter 7.
贊成！「帳號」的翻譯有參考後面的章節，如果這邊要改的話，考慮到一致性，也許後面的章節也一起改？

例如 https://github.com/jlord/git-it/compare/master...zhtw-translation#diff-4124c1036805046a042a8e5cc98b1a4aR10