add more detail about SKIP_EMAIL_VERIFICATION_IN_DEV to e2e tests README

[vincanger]

The signup process is not being verified when running e2e tests on a fresh Open SaaS instance.

• SKIP_EMAIL_VERIFICATION_IN_DEV=true needs to be added to the app/.env.server file to allow the tests to run. This is mentioned in the e2e tests README but could give more detailed instructions