Add CONTRIBUTING.md including Yarn version update policy

[pesho]

There has been some confusion recently regarding when/how Yarn versions should be updated. I think we should agree on a specific strategy and document it somewhere. A CONTRIBUTING.md file seems like a good place for that, especially given that there is a broken link to it in our README.md 😄

The current proposal aims to ensure that a new Yarn release does not cause regressions in an already published SemVer-minor image tag. It's partially motivated by previous issues such as #386. It might be updated after Yarn 1.0 is released.

/cc @nodejs/docker @PeterDaveHello @Daniel15 @bestander for comments.

[pesho]

Oops, forgot about #382 which also adds a CONTRIBUTING.md file. One of the two PRs will have to be updated if the other is merged first.

[PeterDaveHello]

Actually I don't have permission to review here, but personally I prefer to get #382 merged first, unless it's still not acceptable and pending for updates. Otherwise, LGTM 👍

[retrohacker]

We can re-trigger CI, though that would probably be a waste of Travis' resources. I'm 👍 on merging w/o green CI since this is just a doc change.

[Starefossen]

Yeah, I'm fine with merging this without a green CI.

[chorrell]

LGTM too

[SimenB]

Yarn 1.0 is released now. However, I don't see a reason to change our policy on this

https://code.facebook.com/posts/274518539716230

[pesho]

Yarn 1.0 is released now. However, I don't see a reason to change our policy on this

I also like our current policy (which is also very simple to implement in update.sh), but am open to better suggestions.

[chorrell]

Yes I agree.