docs: update readme to describe Steward in details

Signed-off-by: Dmitri Pal <[email protected]>

src/main.rs

@@ -1,6 +1,55 @@
 // SPDX-FileCopyrightText: 2022 Profian Inc. <[email protected]>
 // SPDX-License-Identifier: AGPL-3.0-only
 
+//!
+//! # Overview
+//!
+//! Steward is a critical element of the Confidential Computing infrastructure.
+//! The promise of Confidential Computing is fully utilized when the workload
+//! runtime (Enarx WebAssembly) deployed into a Trusted Execution Environment
+//! (TEE) is assessed and verified for correctness before an actual workload
+//! is released into a TEE from the registry (Drawbridge). An external
+//! **attestation service** must perform evidence verification and assessment
+//! of the hardware's trustworthiness.
+//!
+//! **Steward implements such attestation service in a modular, pluggable
+//! and scalable way.**
+//!
+//! **Modular:** The architecture of the Trusted Execution Environments
+//! significantly differs between hardware vendors. As a result, the content
+//! and structure of the evidence information are vendor-specific. The Steward
+//! employs modular design to process specific types of evidence in different
+//! backends.
+//!
+//! **Pluggable:** Steward employs a pluggable and extensible architecture
+//! to allow the addition of new evidence information to the evidence payload
+//! as well as the support of new hardware architectures.
+//!
+//! **Scalable:** Steward service is stateless. It receives a request with
+//! all the information from the client and makes an assessment. As a result,
+//! it is very lightweight and can be scaled up and down in response to
+//! the request load.
+//!
+//! Attesting the hardware and workload runtime is only one part of
+//! the Steward's responsibility. The other is the translation of the vendor
+//! and use-case-specific attestation evidence into a format that standard
+//! services and interfaces on the Internet can trust. Such a standard is PKI,
+//! so Steward acts as a Certificate Authority that assesses the attestation
+//! evidence and issues a certificate based on this evidence. The certificate
+//! is returned to the workload and used by it to participate in
+//! the authenticated data exchanges with other services over the encrypted
+//! connections.
+//!
+//! # Design Materials
+//!
+//! [Attestation Concept](https://hackmd.io/@enarx/r1Yg2kb_s)
+//! [Attestation Flow](https://hackmd.io/@enarx/SySK2_tHo)
+//! [Full Provisioning Flow with Attestation](https://hackmd.io/@enarx/rJ55urrvo)
+//!
+//! # Licensing and Copyright
+//!
+//! Contributions to this project require copyright assignment to Profian.
+
 #![warn(rust_2018_idioms, unused_lifetimes, unused_qualifications, clippy::all)]
 
 use steward_server::{app, init_tracing, State};