Add Chalearn logo to main page

Summary

Add Chalearn logo to main page:

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge

Remark

We should use "Squash and merge" when merging it to avoid having many useless commits in the main branches.