add notes on how to update the jest snapshots that are used for regression testing of /admin

[jspaaks]

It is my understanding that /admin uses snapshot testing to avoid regressions
https://jestjs.io/docs/en/snapshot-testing

However, I was unable to generate updated snapshots after updating the admin interface. The tests failed because the newly generated app (purposely) differed from the stored snapshots.

So it's probably a good idea to decribe in /admin/README.md when and how to update the snapshots.

[jspaaks]

something like

cd admin
yarn install
yarn test
# (press 'a' to run all tests)
# (press 'w' to show more watch commands)
# (press 'u' to update failing snapshots)

[Tommos0]

yarn test -u should update the snapshots (or pick the 'u' option in the menu)