Document how to work with Sunrise

[lauraluiz]

• [ ] Show screenshot with link to running demo
• [ ] Explain structure
• [ ] Change functionality by DI
• [x] Override templates
• [x] Override CSS
• [x] Override JS
• [ ] Override i18n messages
• [ ] Refresh categories
• [ ] Benefits of replacing parts of template
• [ ] How to set an application secret
• [ ] How to inject credentials for dev/prod/test
• [ ] Describe each related project
• [ ] List minimum requirements regarding project data
  • SKU (without -!)
  • Slug for all supported languages
  • At least one category per product

[lauraluiz]

Related https://github.com/sphereio/sphere-sunrise/issues/10

[lauraluiz]

Related https://github.com/sphereio/sphere-sunrise/issues/14

[lauraluiz]

@mmoelli please list here any other requirement you find