doc: Add introduction text to README

Copy some of the introduction text in the specification to the top-level
README. Also add a link to the PDF.

Signed-off-by: Christophe de Dinechin <[email protected]>

README.md

@@ -1,2 +1,29 @@
-# td-shim
-Confidential Containers Shim Firmware
+# TD-shim - Confidential Containers Shim Firmware
+
+Hardware virtualization-based containers are designed to launch and run
+containerized applications in hardware virtualized environments. While
+containers usually run directly as bare-metal applications, using TD or VT as an
+isolation layer from the host OS is used as a secure and efficient way of
+building multi-tenant Cloud-native infrastructures (e.g. Kubernetes).
+
+In order to match the short start-up time and resource consumption overhead of
+bare-metal containers, runtime architectures for TD- and VT-based containers put
+a strong focus on minimizing boot time. They must also launch the container
+payload as quickly as possible. Hardware virtualization-based containers
+typically run on top of simplified and customized Linux kernels to minimize the
+overall guest boot time.
+
+Simplified kernels typically have no UEFI dependencies and no ACPI ASL
+support. This allows guests to boot without firmware dependencies. Current
+VT-based container runtimes rely on VMMs that are capable of directly booting
+into the guest kernel without loading firmware.
+
+TD Shim is a simplified [TDX virtual firmware](doc/tdshim_spec#vfw) for the
+simplified kernel for TD container. This document describes a lightweight
+interface between the TD Shim and TD VMM and between the TD Shim and the
+simplified kernel.
+
+## Documents
+
+* [Introduction (PDF)](doc/td-shim introduction.pdf)
+* [TD-Shim specification](doc/tdshim_spec.md)