Update the design principles in the README.md

[sunfishcode]

While the original goals of capability-based security and interposability remain, these goals have been factored out into the component model, so we now update WASI to refer to the component model rather than defining these concepts itself.

[sunfishcode]

I believe I've addressed all the comments raised so far. Are there any further comments here?

[sunfishcode]

All the comments here are addressed. For any further questions or concerns, please file new issues!