Start the documentation off with some common use cases

[nealmcb]

Few people are fully versed in the complex, intersecting worlds of secrets, encryption, HSMs, cloud services, containers etc.

So at the very top of the GitHub README, Quick Start, etc, it would help to link to, or provide, some "level-setting" information to give some background and note common use cases for Custodia, perhaps combined with cautionary tales and horror stories about folks who did it wrong.

I'm still looking myself, but it seems that the resources at #222 are probably helpful in this regard, as is Secrets and LIE-abilities: The State of Modern Secret Management (2017) – On Docker – Medium.