Create or point to pre-exercise ROOT

[caredg]

Create some instructions on how to access ROOT and how to use it. This is linked to cernopendata/cms-opendata-guide#23

[caredg]

I am a bit worried about the HTT example needing ROOT with version > 6.16. There is a nice root intro in the cmsdas repository already in place. I am not sure how we will get v6.16 or greater in an easy way? Does @Someone have a clear view on this?

[caredg]

Ok, my bad. I have tested (I forgot if I already did before, I guess I did) these instructions and the root setup from cvmfs works just fine (in a container with access to host cvmfs client). So I presume it is the same in the VM.

So, I believe we could very well use (actually directly pointing to some episodes) some sections in this lesson from cmsdas. The RDataFrame episode, for example (the one I checked), even uses an open data file, so it works beautifully.

[caredg]

Obviously, the setup and/or Get ROOT needs modification according to cms open data setup and not internal cms.
BTW, the entire skimming took 35 in on my i7 desktop.