Security doc + status analysis for the SDK

Wrote up the SECURITY.md plus a "current status" analysis. The status analysis is the doc I wish more crypto SDKs shipped: an honest table of what's audited, what's not, and what's "implemented but please don't run this in production yet". The threat model section is short. Three lines: The SDK assumes the host machine is not compromised. The SDK does not protect against rubber-hose cryptanalysis.…