README for running rate-estimation tools with NanoAOD

[RobertJWard]

Update rate-estimation README with instructions on how to run with nanoAOD, with some notes on what options it should be used with.

TODO:

• Add an example list of nanoAOD files equivalent to ntuple/Run3_NuGun_MC_ntuples.list for people to run quick tests locally
• Add example nanoAOD run command to the "test run" section (with new NuGun list)