Add note about CI usage

[seancorfield]

For folks in a CI pipeline that want a cache with pre-downloaded dependencies, they need to run -P with the same command-line they would use to run depstar to get depstar itself and t.d.a

This would be in addition to the project dependencies that they plan to pass into depstar, i.e., if they tell depstar an alias, they should also use that alias for the "prepare" operation.