Improve README.md with more information

[jsmanrique]

By looking at the README.md I don't get what this projects does or solves, how is it related with other tools, how can I contribute, or how it works once deployed. It says it is an Administrative interface for GrimoireLab`, but I don't know what and how it administrates anything in GrimoireLab (data sources?, panels?, users?)

Wouldn't it be nice having such information in the README.md to understand the goals of the project and what to expect from it?