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 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
修改了部分翻译，出现的比较多的paper form觉得翻译成纸质表格更好；
ripe complete, page 50 - 54
Merge branch 'gh-pages' of https://github.com/johnstart/data-science.git into develop
Merge pull request #15 from jue-jiang/develop

Develop
Merge pull request #16 from ripeconan/develop

Develop