write chapter 8:abstructs of tag and branch
write chapter 8:create default folder structure
Merge branch 'chapter-8-default-folder-structure'
modify chapter 7:add navigation to next chapter
write chapter 8:add links navigating to headers
modify-readme:add navigation to chapter 8
Merge branch 'modiry-chapter-7'
Merge branch 'add-navigation-of-toward-chapter-8-to-readme'