Fix image link, add link to license & linting (fix-cla)

[stamp-cmd]

This is documentation changes. Note: This version is misname cla error

• [x] Fix image link from master https://github.com/facebook/duckling/raw/master/logo.png to https://github.com/facebook/duckling/raw/main/logo.png

• [x] Add language to code fences

• [x] Add link to LICENSE file

[facebook-github-bot]

@stroxler has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[stroxler]

@stamp-cmd thanks for the changes, I'll try to get this merged shortly!