Nice to have: enhance user-created repo

[nicolocarpignoli]

Nice to have on the user repo, created with AR.js Studio:

• set Github Pages as 'default' branch
• on the Readme, add the URL of github pages
• output Github repository URL (not only github pages) and add it on the response of the package.serve when packageType is github
• show marker image on the Readme (using relative path in reference of the repo).