説明文を補強。
Merge branch 'master' into step-00
Merge branch 'step-00' into step-01
Merge branch 'step-01' into step-02
Merge branch 'step-02' into step-03
Merge branch 'step-03' into step-04
Merge branch 'step-04' into step-05
Merge branch 'step-05' into step-06
Merge branch 'step-06' into step-07
Merge branch 'step-07' into step-08
Merge branch 'step-08' into step-09
Merge branch 'step-09' into step-10
Merge branch 'step-10' into step-11
Merge branch 'step-11' into step-12
Merge branch 'step-12' into step-13
Merge branch 'step-13' into step-14
Merge branch 'step-14' into step-15