Update README.md, Nondeterminism.md, and Security.md

Rewrite README.md to reflect the design repo's current purposes and update outdated information. Also, remove text from README.md which now lives in the webassembly.org website, and link to the website instead. Also, update some outdated information and links in Nondeterminism and Security.md, so that we can link to them as examples of fairly current documents.