Adds a step to the "Project settings" section of `docs/branching/README.md` instructing the person doing the branch cut to disable the waterfall in the non-staging branch's Evergreen project settings (e.g. https://spruce.corp.mongodb.com/project/mongodb-mongo-v9.0/settings/general).

While branching changes are developed and merged through `vX.Y-staging`, the `vX.Y` (non-staging) project should not schedule waterfall builds. The step documents setting "Waterfall" to "Disabled" under the "Repotracker" section of General Settings, and re-enabling it once the staging branch is merged and the branch is officially released.

Docs-only change; no code or testing impact.