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 

[caicancai]

@drmingdrmer Hi, I found that my previous mention in the main branch seemed a little unreasonable, so I re-mentioned a pr, for which I am sorry

[drmingdrmer]

It has come to my attention that the README file is also utilized on docs.rs. Therefore, using local links could lead to broken links on the docs.rs platform.

[caicancai]

It has come to my attention that the README file is also utilized on docs.rs. Therefore, using local links could lead to broken links on the docs.rs platform.

Thanks for reminding me. I'll change the official link

[drmingdrmer]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @caicancai)

[caicancai]

I seem to have typed one space less, and I'm sorry for that