Update documentation links, convert README to markdown.

While ASCIIDOC is richer, it isn't as widely supported.
Additionally, the image used for mangos previously is not
one we have ownership for, so we feel best to drop it.
(We will follow up with a proper logo later.)

README.md

@@ -0,0 +1,89 @@
+# mangos™ v3
+
+
+[![Linux](https://img.shields.io/github/workflow/status/nanomsg/mangos/linux?logoColor=grey&logo=linux&label=)](https://github.com/nanomsg/mangos/actions)
+[![Windows](https://img.shields.io/github/workflow/status/nanomsg/mangos/windows?logoColor=grey&logo=windows&label=)](https://github.com/nanomsg/mangos/actions)
+[![macOS](https://img.shields.io/github/workflow/status/nanomsg/mangos/darwin?logoColor=grey&logo=apple&label=)](https://github.com/nanomsg/mangos/actions)
+[![Coverage](https://img.shields.io/codecov/c/github/nanomsg/mangos?logoColor=grey&logo=codecov&label=)](https://codecov.io/gh/nanomsg/mangos)
+[![Discord](https://img.shields.io/discord/639573728212156478?label=&logo=discord)](https://discord.gg/wewTkby)
+[![Documentation](https://img.shields.io/badge/godoc-docs-blue.svg?label=&logo=go)](https://pkg.go.dev/go.nanomsg.org/mangos/v3)
+[![License](https://img.shields.io/github/license/nanomsg/mangos.svg?logoColor=silver&logo=opensourceinitiative&label=&color=blue)](https://github.com/nanomsg/mangos/blob/master/LICENSE)
+[![Version](https://img.shields.io/github/v/tag/nanomsg/mangos?logo=github&sort=semver&label=)](https://github.com/nanomsg/mangos/releases)
+
+
+
+_Mangos™_  is an implementation in pure Go of the *SP*
+(`Scalability Protocols`) messaging system.
+These are colloquially  known as `nanomsg`.
+
+> *NOTE*: The import path has changed! Please change any references
+to `go.nanomsg.org/mangos/v3`.
+The old v2 imports will still work for old applications, provided that
+a sufficiently modern version of Go is used.  However, no further work
+will be done on earlier versions.
+Earlier versions will still inter-operate with this version, except that
+within the same process the `inproc` transport can only be used by
+consumers using the same version of mangos.
+
+The modern C implementation of the SP protocols is available as
+[NNG™](https://github.com/nanomsg/nng).
+
+The original implementation of the SP protocols is available as
+[nanomsg™](http://www.nanomsg.org).
+
+Generally (modulo a few caveats) all of these implementations can inter-operate.
+
+The design is intended to make it easy to add new transports,
+as well as new topologies (`protocols` in SP parlance.)
+
+At present, all the Req/Rep, Pub/Sub, Pair, Bus, Push/Pull, and
+Surveyor/Respondent patterns are supported.
+This project also supports an experimental protocol called Star.
+
+Supported transports include TCP, inproc, IPC, WebSocket, WebSocket/TLS and TLS.
+
+Basic interoperability with nanomsg and NNG has been verified (you can do
+so yourself with `nanocat` and `macat`) for all protocols and transports
+that _NNG_ and _nanomsg_ support, except for the _ZeroTier_ transport and the PAIRv1
+protocol, which are only supported in _NNG_ at this time.
+
+There are a number of projects that use these products together.
+
+## Documentation
+
+For API documentation, see https://pkg.go.dev/go.nanomsg.org/mangos/v3.
+
+## Testing
+
+This package supports internal self tests, which can be run in
+the idiomatic Go way.
+(Note that most of the tests are in a test subdirectory.)
+
+    $ go test go.nanomsg.org/mangos/v3/...
+
+There are also internal benchmarks available:
+
+    $ go test -bench=. go.nanomsg.org/mangos/v3/test
+
+## Commercial Support
+
+[Staysail Systems, Inc.](mailto:[email protected]) offers
+[commercial support](http://staysail.tech/support/mangos) for mangos.
+
+## Examples
+
+Some examples are posted in the directories under `examples/` in this project.
+
+These examples are rewrites (in Go) of Tim Dysinger's
+[Getting Started with Nanomsg](http://nanomsg.org/gettingstarted/index.html).
+
+Running `go doc` in the example directories will yield information about how
+to run each example program.
+
+Enjoy!
+
+______
+
+Copyright 2021 The Mangos Authors
+
+mangos™, Nanomsg™ and NNG™ are [trademarks](http://nanomsg.org/trademarks.html) of Garrett D'Amore.