Add changelog entry for prototype kit config build

[colinrotherham]

This PR adds a changelog entry for:

• Support Prototype Kit v13 #2851

Closes #2889

[36degrees]

I'm not sure it's that useful for users to know how this works in this much detail, and arguably this doesn't really tell them what govuk-prototype-kit.config.json actually does.

I wonder if we should simplify to something like

In pull request 2851: Supporting v13 of the Prototype Kit we introduced support for the plugins system introduced in the upcoming Prototype Kit v13.

It's a bit repetitive given the PR title…. I wonder if we could just change the title of the PR to 'Support Prototype Kit v13' and then just list this with the rest of the fixes below?

@claireashworth do you have any thoughts on this one?

[colinrotherham]

@36degrees Yeah that's sensible, I've pushed up your first suggestion

At least two other ### Fixes sections included the format:

In [pull request XXXX: Description…]

So I pinched it 😊

If the fixes list is a better place, let me know

In terms of what it does, I knew this bit was further down. If it helps at all?

## 2.7.0 (Feature release)

🆕 New features:

- Added config file for prototype kit.

  Added a configuration file for an experimental feature in Prototype kit to allow departments to use their own frontend alongside govuk-frontend

  Thanks @matcarey (HMRC) for this contribution

  ([PR #1102](https://github.com/alphagov/govuk-frontend/pull/1102))

Cheers for the milestone tip 👍

[36degrees]

Removed this from the milestone as the linked issue is already associated (so it's effectively represented in the milestone twice)

[claireashworth]

Suggested change

	In [pull request 2851: Support Prototype Kit v13](https://github.com/alphagov/govuk-frontend/pull/2851) we introduced support for the plugins system introduced in the upcoming Prototype Kit v13.
	In [pull request 2851: Support Prototype Kit v13](https://github.com/alphagov/govuk-frontend/pull/2851) we introduced support for the plugins system included in the upcoming Prototype Kit v13.

[colinrotherham]

Nice spot, double "introduced"! Pushed it up

@36degrees @claireashworth Good to go?