updated CONTRIBUTING.md

Hi,

I have simplified the CONTRIBUTING.md:

        https://github.com/pharo-project/pharo/blob/Pharo8.0/CONTRIBUTING.md


- removed all the pre-git infos.
- How to propose a bug fix or enhancement now points to
                https://github.com/pharo-project/pharo/wiki/Contribute-a-fix-to-Pharo

I changed the “Welcome” ProBot message to now point to these links when someone opens their very first issue.

        Marcus