Request create a contribuition guide.

[luisreinoso]

Hi, I would like to contribute to primeng but I think so that actually is not sufficient information about this. I propose this ideas.

- Add some information to setup a development environment
- Add other label issues like PR-WELCOME, EASY, NEXT-RELEASE, NEED-HELP on github.
- Add roadmap to know what is the direction of project.

I think that community could to help to do a better primeng.