Add proper README.md

[fperez]

Should include

• [ ] What is the project: short description of purpose
• [ ] License notice (actual license is in COPYING)
• [ ] Installation instructions from packages
• [ ] Installation instructions from source
• [ ] Link to cesium_web and clarify distinction
• [ ] Link to examples page on website

[arkwave]

@jiesuncal and myself are working on this!

[stefanv]

@arkwave Great! Please move this item from 'TODO' to 'In Progress' in the projects tab.