Update index.md

1.添加如何写markdown表格说明
2.文件提交说明
P50, almost done
Merge branch 'master' into gh-pages
Merge branch 'master' into gh-pages
Merge pull request #9 from chenyuqing/develop

chap03_第二次pull
Merge branch 'master' into gh-pages
Page 51 almost done
complete Page 50 and 51
Merge branch 'gh-pages' of https://github.com/johnstart/data-science.git into develop
Merge pull request #10 from ripeconan/develop

Develop
task.md
Merge pull request #11 from hanlu-chen/develop

task.md
test
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draft
update title
update title