message: Add complement of information to README
  based on current v1.2 README plus information shared in issue tickets.
  TODO: explain how to test / lint the library in section `contributing`. Command `build` or `dist`?
diff: # Sigma v2
  +Sigma is a JavaScript library dedicated to graph drawing, mainly developed by [@jacomyal](https://github.com/jacomyal) and [@Yomguithereal](https://github.com/Yomguithereal).
  +
  +## Overview
  +
  +[sigmajs.org website](http://sigmajs.org) provides a global overview of the project.
  +
  +As of version `v2`, `sigma` focuses on the management of graph display: layout, rendering, interaction... The graph model is managed in a separate library called `graphology`, which is packed with convenience methods to manage graph data structures.
  +
  +A set of demo examples contain various use-cases that might help you understand how to use sigma (read further below).
  +
  +## Installation
  +
  +Sigma is a javascript library, it is available in `npm` package manager.
  +
  + npm install sigma
  +
  ## Examples
  A development server can be spawned locally to view the examples. Visit `localhost:8000` after executing the following commands:
  @@ -8,3 +24,7 @@ A development server can be spawned locally to view the examples. Visit `localho
  npm run examples
  You can play around with the files in directory `examples`, the web pages are live-reloaded whenever the code gets changed.
  +
  +## Contributing
  +
  +You can contribute by submitting [issues tickets](http://github.com/jacomyal/sigma.js/issues) and proposing [pull requests](http://github.com/jacomyal/sigma.js/pulls). Make sure that tests and linting pass before submitting any pull request.
  