Doc: add license link

[caicancai]

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[drmingdrmer]

Thank you man.

The commit message should starts with Doc: not Doc :. The pull request title does not matter.

You could fix it with git commit --amend to update the commit message and git push --force to update the remote branch.

[caicancai]

Thank you man.

The commit message should starts with Doc: not Doc :. The pull request title does not matter.

You could fix it with git commit --amend to update the commit message and git push --force to update the remote branch.

Thank you for reminding me