Refactor CONTRIBUTING.md across BIDS repos

[Remi-Gau]

Opening this issue here as it seems to be the biggest CONTRIBUTING of the lot.

This is part of the effort to reorganize our several wesbites and streamline things a bit.

The goal is to have the most of the CONTRIBUTING sections that can be common to many if not all repos (see some examples below) and to put them all in the single CONTRIBUTING.md in the repo https://github.com/bids-standard/.github and point to it.

• https://github.com/bids-standard/bids-specification/blob/master/CONTRIBUTING.md#contributing-through-github
• https://github.com/bids-standard/bids-specification/blob/master/CONTRIBUTING.md#understanding-issues
• https://github.com/bids-standard/bids-specification/blob/master/CONTRIBUTING.md#making-a-change-with-a-pull-request
• ...

The contributing from each repo could then just hold the things that are specific to it.