docs: make it clearer that packageManager should be set as part of …

…the upgrade to use yarn

CHANGELOG.md

@@ -22,7 +22,8 @@ Changes since the last non-beta release.
 
 - Use `package_json` gem to manage Node dependencies and commands, and use `npm` by default [PR 430](https://github.com/shakacode/shakapacker/pull/430) by [G-Rath](https://github.com/g-rath)
 
- This enables support for package managers other than `yarn`, with `npm` being the default; see the readme for more details.
+ This enables support for package managers other than `yarn`, with `npm` being the default; to continue using Yarn,
+ specify it in `package.json` using the [`packageManager`](https://nodejs.org/api/packages.html#packagemanager) property.
 
 - Remove `yarn_install` rake task, and stop installing js packages automatically as part of `assets:precompile` [PR 412](https://github.com/shakacode/shakapacker/pull/412) by [G-Rath](https://github.com/g-rath).