Add an initial list of BMI design principles

[mcflugen]

I thought it would be good to have a list of BMI design principles in the README. I'll list them here as well,

BMI Design Principles

• Easy to implement. Preference goes to the implementer over the end user.
• Self describing.
• Common tasks are left up to a framework.
• Popeye Principle (“I am what I am”). BMI libraries just report about themselves.
• Language agnostic
• NOT intended to accommodate every model and use case.
• Intended for model coupling but not necessarily model-to-model coupling.
• A model can be a numerical software model but could also be a dataset.
• Does not inject itself into a model.
• Minimal but can be open to extensions.

[mdpiper]

These should be enumerated, like commandments.