Update qrcode.markdown

将水印移到正中
Merge pull request #5 from SunRunAway/develop

Update qrcode.markdown
upload api add callback desc
Merge pull request #6 from why404/feature/upload_api_add_callback_desc

upload api add callback desc
link
Merge pull request #7 from why404/feature/upload_api_add_callback_desc

link