Added some instructions to generate the rntuple files

dev/make-root/Run2012BC_DoubleMuParked_Muons_rntuple_1000evts.md

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+First, download `Run2012BC_DoubleMuParked_Muons.root` from [here](https://opendata.cern.ch/record/12341). Then, run the following commands in ROOT.
+
+```cpp
+auto importer = ROOT::Experimental::RNTupleImporter::Create("./Run2012BC_DoubleMuParked_Muons.root", "Events", "./Run2012BC_DoubleMuParked_Muons_rntuple_1000evts.root");
+auto c = importer.get();
+c->SetMaxEntries(1000);
+c->Import()
+```

dev/make-root/test_ntuple_extension_columns.md

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+1. Checkout an Athena build with ROOT nightly (currently has RNTuple RC2)
+2. Get an ATLAS RAW file
+3. Run reco:
+```bash
+Reco_tf.py \
+  --CA="True" \
+  --maxEvents=40 \
+  --multithreaded="True" \
+  --sharedWriter="True" \
+  --parallelCompression="False" \
+  --inputBSFile="raw.root" \
+  --outputDAOD_TLAFile="daod_tla.root" \
+  --preExec="flags.Output.StorageTechnology.EventData=\"ROOTRNTUPLE\"";
+```