Update changelog

CHANGELOG.md

@@ -3,6 +3,10 @@
 ## Unreleased
 
 ### Breaking changes
+- [#1638: Make serve default command](https://github.com/alphagov/govuk-prototype-kit/pull/1638)
+  - Running `npm start` after creating starter prototype will now run 'production' command
+  - Users now need to run `npm run dev` when they want to start their prototype on their local machine
+  - We try and warn users in some circumstances where we think they may have run `npm start` accidentally
 - [#1617: Making most files optional](https://github.com/alphagov/govuk-prototype-kit/pull/1617) This alongside other work we've been doing allows users of the kit to delete files they're not using.
 - [#1589: Create management pages](https://github.com/alphagov/govuk-prototype-kit/issues/1589) Providing pages for the user to manage their prototype.
 - [#1615: Removing GOVUK Frontend specific integration](https://github.com/alphagov/govuk-prototype-kit/pull/1615) GOV.UK Frontend now integrates in the same way as any other plugin can.  We're allowing SASS settings to be set before the plugins run if they're put in app/assets/sass/settings.scss.