Add 'Unreleased' section to changelog; update 'contributing' docs

[flarnie]

what is the change?:

• Add 'Unreleased' section to CHANGELOG
• Add step to 'contributing' about updating the CHANGELOG

why make this change?:

Whoever does the release of a new React version must manually read
through the recent git history and write the CHANGELOG update.

If we incrementally add to the CHANGELOG under the 'Unreleased' header,
as we make PRs, then to release a new version will just be updating
'Unreleased' to be the new version number and adding a blank section for
'Unreleased'.

See http://keepachangelog.com/en/0.3.0/ for more info about this idea.

test plan:
Visual verification of the change, and also I built the website locally
and inspected the change.

issue:
#9527

[flarnie]

I think the next step is to start back-filling the 'Unreleased' section, as well as adding the notes from the 15.5.* updates and for the upcoming 15.6 update. I'll work on that in a separate PR.

[flarnie]

Thanks @gaearon - since you are so active in reviewing PRs I am reassured that you have no issues with this change.

[gaearon]

We'll need to work out how this works with:

• Major / minor releases (e.g. do we put breaking changes there)
• Visibility: this section can get large and confuse people who expect the info about latest release there. Maybe worth collapsing it somehow (does <details> tag work in Markdown?)

[flarnie]

Major / minor releases (e.g. do we put breaking changes there)

I'm not sure this affects our decisions about where to put breaking changes. You are right that our current situation makes things complicated - we have v15.6 and the continuing snowball of 16.0, both in development at the same time. Here is my thought for that:

## [Unreleased]
// ... Anything that is on master but not cherry-picked to 15.*

## 15.6 [Unreleased]
// ... I will backfull the changelog and continue updating it as we merge things into 15.6, then remove the '[Unreleased]' part when releasing this.

## 15.4
// ... the changelog for 15.4

// ... rest of the CHANGELOG

When something is merged to master, we add a line under 'Unreleased'. And that's how it will work post 16.0, when there is just one branch and we release from master.

For now, when something gets cherry-picked from master I would move the line from 'Unreleased' to '15.6 [Unreleased]'.

[flarnie]

Current plan for this:

1. Land Update Changelog for v15.5.1-15.5.4 #9537
2. Backfill a section for '15.6.0 [Unreleased]' and also collapse that in a 'details' tag. Edit: - this pr - Add 'unreleased' incremental change log for v15.6.0 #9544
3. Update this PR to backfill at least the basics of what is currently on master and not on the 15.6-dev branch.
4. Land this.

[gaearon]

When we change this we usually also want to change the GitHub pull request template (unless you already have). It's in .github folder.

[flarnie]

Thinking of revisiting this now that we are getting close to the 16.0 RC. Will come back to it after wrapping up any bug fixes I can help with.

[flarnie]

Closing this in favor of #10730