added comments
add draft chapter2
write chaper6
Merge pull request #8 from shouheiumemoto/chapter1

第1章の草案を追加
Merge pull request #6 from seiyawada/chapter6

6章の草案を書きました。
Merge pull request #5 from TasukuNakano/chapter2

第２章の草案をコメントアウトで追加しました(再送)
Merge pull request #9 from KotaroKimura/chapter4

added comments
write chapter1