Document external/ directory

[watson-ij]

Can we add a README or wiki page that documents where the code we've taken comes from, and if/what we changed (mostly I think this was just splitting the header and cxx)

[watson-ij]

Also, we should document where the procedure we follow for scale factors comes from.

[jshlee]

i prefer adding to wiki (https://github.com/CPLUOS/nano/wiki)

[tt8888tt]

I guess this issue can be closed? unless there're more things you want it to be added.

[watson-ij]

Well, once its documented where the files came from. We have a wiki for data but not yet externals or?

[jshlee]

is this finished? can we close?

[watson-ij]

No one ever updated the wiki with externals information @YeChanKang @DanielLee1004

[quark2]

First strike on the documentation: See [1]. Only lumiTools and pileupTool has been documented. The remainings will be filled.

[1] https://github.com/CPLUOS/nano/wiki/Usage-of-external-codes#how-to-set-the-numbers-in-this-module