Update Github readme

[sunix]

The README should explain

• What does the project do, the main features
• have few screenshots of the plugin
• how to build the project
• how to dev with the project (Eclipse + Intellij)
• the global architecture

[mleduque]

I don't think the architecture should be in the readme.
It can be linked from the readme, but it should probably be in another .md file. Actually, the same could be said for "how to dev".