Note origin of uploaded videos
Note origin of uploaded videos

Note origin and license of videos uploaded in https://github.com/flutter/assets-for-api-docs/pull/13
flutter/assets-for-api-docs,flutter/assets-for-api-docs,flutter/assets-for-api-docs,flutter/assets-for-api-docs,flutter/assets-for-api-docs,flutter/assets-for-api-docs