Adding ClearlyDefined

[emmairwin]

I know with other metrics we propose tools, suggest adding https://clearlydefined.io/about

[germonprez]

As per a comment from @GeorgLink:

The ClearlyDefined project defined a "License Clarity" metric, that we can incorporate in CHAOSS.

https://github.com/clearlydefined/license-score/blob/master/ClearlyLicensedMetrics.md

I would not copy this design page 1-to-1, but create a short summary to be included in CHAOSS and then link for the implementation details to ClearlyDefined.