Add a basic README

Author: nc6

README.md

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+<h1 align="center">Cuddle</h1>
+
+Cuddle is a library for generating and manipulating [CDDL](https://datatracker.ietf.org/doc/html/rfc8610). 
+
+<p align="center">
+  <a href="https://github.com/input-output-hk/cuddle/actions/workflows/ci.yml">
+    <img alt="GitHub Workflow Status (master)" src="https://img.shields.io/github/actions/workflow/status/input-output-hk/cuddle/ci.yml?branch=master&style=for-the-badge" />
+  </a>
+</p>
+
+## Supported features
+
+Cuddle currently supports the following CDDL features:
+
+- Groups
+- Values
+- Choices
+- Maps
+  - Structs
+  - Tables
+- Predefined CDDL types
+- Tags
+- Unwrapping
+- Sockets/plugs
+- Generics
+
+### Partial support
+
+- Representation types
+  - Representation types are correctly parsed and formatted, but only certain 
+    types are understood in CBOR generation.
+- Cuts
+  - Cut syntax is parsed and formatted, but ignored for CBOR generation.
+- Controls
+  - Controls are correctly parsed and formatted, but only certain controls are
+    understood in CBOR generation.
+- Operator precedence
+- Comments
+  - Comments are currently stripped during formatting.
+
+### Unsupported
+
+- Escaping in bytestrings
+
+
+## The cuddle tool
+
+Included in this package is a command line tool for working with CDDL files. It
+currently supports three functions:
+
+- Formatting of CDDL files
+- Validating that a CDDL file is legal
+- Generating random CBOR terms matching CDDL productions.
+
+# Huddle
+
+One of the principal features of Cuddle is the ability to define your CDDL in a 
+Haskell DSL, called Huddle. This offers the following benefits:
+
+- Ability to use Haskell's abstraction facilities to define more complex CDDL 
+  constructions.
+- Some measure of compile-time safety for your CDDL. Attempting to reference
+  an undefined identifier will be a compile-time error, for example.
+- The ability to modularise your configuration. CDDL lacks any real facility
+  for splitting a CDDL spec amongst multiple files. We solve this instead using
+  Haskell's module system.
+
+Obviously, this comes with the downside of needing to sensibly mesh the 
+different abstraction facilities offered by Haskell and CDDL. We have tried to 
+find a balance where the Huddle code roughly matches the CDDL but gains many
+of the advantages of writing in Haskell.