a few updates to contributing guide

[govekk]

Added a bit of context info to contributing guide (e.g. syntax, pr preview links)

[github-actions]

Automated Review URLs

• Readthedocs
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[govekk]

Becoming more:

• Update contributors to be OME-relevant
• Update Readme to be more relevant to doc contributors now that spec and RFC are being moved out into separate repos
• Update license
• linking to github from webpage more openly

[dstansby]

Given everyone up to now has contributed under the W3C Software and Document License, and as far as I know all contributors retain the copyright of their contributions, doesn't changing the license require the explicit agreement of every contributor to this repository?

[joshmoore]

Thanks for the care here, @dstansby. In the heat of the hackathon, I wasn't cautious enough. In fact, there may be differing license statements that will need clarifying. I'll revert the change here (and leave @jo-mueller to do the same in ngff-spec re: ome/ngff-spec#27) and then we can re-evaluate once 0.6 is out the door.