Explanations of complexity and churn on Readme / default value of complexity

[medic-code]

Hi, I wondered whether a few updates to the readme might be useful.

A one line explanation of the different complexity metrics we can choose in the package (A rational for those metrics might be quite nice too) might make it slightly easier for those who don't know much about complexity metrics to understand the package.

Also just a small thing I noticed on the readme, there's no where it says what the default strategy for complexity is if we don't explicitly set it. Reading the code I noticed it was sloc and I suspect you can guess this when you run it for the first time but wondered whether its worth being explicit about this?