Add initial readme

Author: jbowes

README.md

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+<!--
+  Attractive html formatting for rendering in github. sorry text editor
+  readers! Besides the header and section links, everything should be clean and
+  readable.
+-->
+<h1 align="center">httpsig</h1>
+<p align="center"><i>Standards-based HTTP request signing and verification for<a href="https://golang.org">Go</a></i></p>
+
+<div align="center">
+  <a href="https://godoc.org/github.com/jbowes/httpsig"><img src="https://godoc.org/github.com/jbowes/httpsig?status.svg" alt="GoDoc"></a>
+  <img alt="Alpha Quality" src="https://img.shields.io/badge/status-ALPHA-orange.svg" >
+  <a href="./LICENSE"><img alt="BSD license" src="https://img.shields.io/badge/license-BSD-blue.svg"></a>
+  <a href="https://goreportcard.com/report/github.com/jbowes/httpsig"><img alt="Go Report Card" src="https://goreportcard.com/badge/github.com/jbowes/httpsig"></a>
+</div><br /><br />
+
+## Introduction
+
+`httpsig` provides support for signing and verifying HTTP requests according
+to the [Signing HTTP Messages][msgsig] draft standard. This standard focuses
+on signing headers and request paths, and you probably want to sign the
+request body too, so body digest calculation according to
+[Digest Headers][dighdr] is included.
+
+## The Big Feature Matrix
+
+This implementation is based on version `05` of "Signing HTTP Messages" 
+(`draft-ietf-htttpbis-message-signatures-05`). Digest computation is based on
+version `05` of "Digest Headers" (`draft-ietf-httpbis-digest-headers-05`).
+
+## Contributing
+
+I would love your help!
+
+`httpsig` is still a work in progress. You can help by:
+
+- Opening a pull request to resolve an [open issue][issues].
+- Adding a feature or enhancement of your own! If it might be big, please
+  [open an issue][enhancement] first so we can discuss it.
+- Improving this `README` or adding other documentation to `httpsig`.
+- Letting [me] know if you're using `httpsig`.
+
+
+<!-- Other links -->
+[go]: https://golang.org
+[msgsig]: https://datatracker.ietf.org/doc/draft-ietf-httpbis-message-signatures/
+[dighdr]: https://datatracker.ietf.org/doc/draft-ietf-httpbis-digest-headers/
+
+[issues]: ./issues
+[bug]: ./issues/new?labels=bug
+[enhancement]: ./issues/new?labels=enhancement
+
+[me]: https://twitter.com/jrbowes