Add Technical Charter to repo

[lucian-balea]

• Do you want to request a feature or report a bug?

feature

• What is the current behavior?

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

• What is the expected behavior?

Add attached final technical charter to powsybl repository:
Technical Charter - LF Energy powsybl FINAL April-2019.pdf

And reference to the file in CONTRIBUTING:
This project is governed by the [PowSyBl Technical Charter](link_to_file)

• What is the motivation / use case for changing the behavior?

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)

[mathbagu]

This has been taken into account in the new version of the website that will be published this summer