Merge pull request #4021 from alphagov/v4-stats-docs

Add README for stats package

shared/stats/README.md

@@ -0,0 +1,17 @@
+# GOV.UK Frontend stats
+
+This package is used to generate Rollup visualizer stats for built GOV.UK Frontend modules.
+
+```shell
+npm run build:stats --workspace govuk-frontend
+```
+
+Stats are shown in the review app [deployed to Heroku](https://govuk-frontend-review.herokuapp.com) or when run locally using `npm start`.
+
+## Stats for previous version
+
+GOV.UK Frontend is resolved from [./packages/govuk-frontend](../../packages/govuk-frontend/) unless a previous version is installed locally:
+
+```shell
+npm install [email protected] --save --workspace govuk-frontend-stats
+```