Provide suitable README files for CB7, GDCC benchmarks

[davidlmobley]

The CB7 and GDCC benchmarks do not have a README providing data or references to other computational works. We should provide these since (a) it's the right thing to do, and (b) we want to be consistent with the other data sets.

(This was mentioned in #47 here #47 (comment) but I'm creating an issue so we don't forget.)