Update to contributing.md

I have been working on an updated version of our contributing document. You can find the PR here: https://github.com/w3c/distributed-tracing-wg/pull/6.  Sergey already provided a couple of comments. I invite the other to provide comments as well as I want to get this approved for our next meeting.

Philippe, there are some parts that require your attention as they also relate the W3C best practices.

Please try to provide your input by the end of this week.

// Alois


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