doc: in v3 branch's README, point to v4 as latest (#101)

Also:

 - move the Versions stanza to the top
 - remove the Help Wanted
 - remove the doc / build badges

Since this is not the main branch, it will mainly be viewed on
pkg.go.dev after we make a patchlevel release.

README.md

@@ -1,18 +1,22 @@
 # Go JOSE
 
-[![godoc](https://pkg.go.dev/badge/github.com/go-jose/go-jose/v3.svg)](https://pkg.go.dev/github.com/go-jose/go-jose/v3)
-[![godoc](https://pkg.go.dev/badge/github.com/go-jose/go-jose/v3/jwt.svg)](https://pkg.go.dev/github.com/go-jose/go-jose/v3/jwt)
-[![license](https://img.shields.io/badge/license-apache_2.0-blue.svg?style=flat)](https://raw.githubusercontent.com/go-jose/go-jose/master/LICENSE)
-[![test](https://img.shields.io/github/checks-status/go-jose/go-jose/v3)](https://github.com/go-jose/go-jose/actions)
+### Versions
+
+[Version 4](https://github.com/go-jose/go-jose)
+([branch](https://github.com/go-jose/go-jose/),
+[doc](https://pkg.go.dev/github.com/go-jose/go-jose/v4), [releases](https://github.com/go-jose/go-jose/releases)) is the current stable version:
+
+    import "github.com/go-jose/go-jose/v4"
+
+The old [square/go-jose](https://github.com/square/go-jose) repo contains the prior v1 and v2 versions, which
+are deprecated.
+
+### Summary
 
 Package jose aims to provide an implementation of the Javascript Object Signing
 and Encryption set of standards. This includes support for JSON Web Encryption,
 JSON Web Signature, and JSON Web Token standards.
 
-**Help Wanted!** If you'd like to help us develop this library please reach
-out to css (at) css.bio. While I'm still working on keeping this maintained,
-I have limited time for in-depth development and could use some additional help.
-
 **Disclaimer**: This library contains encryption software that is subject to
 the U.S. Export Administration Regulations. You may not export, re-export,
 transfer or download this code or any part of it in violation of any United
@@ -39,17 +43,6 @@ of [case-insensitive matching](https://www.ietf.org/mail-archive/web/json/curren
 This is to avoid differences in interpretation of messages between go-jose and
 libraries in other languages.
 
-### Versions
-
-[Version 3](https://github.com/go-jose/go-jose)
-([branch](https://github.com/go-jose/go-jose/tree/v3),
-[doc](https://pkg.go.dev/github.com/go-jose/go-jose/v3), [releases](https://github.com/go-jose/go-jose/releases)) is the current stable version:
-
-    import "github.com/go-jose/go-jose/v3"
-
-The old [square/go-jose](https://github.com/square/go-jose) repo contains the prior v1 and v2 versions, which
-are still useable but not actively developed anymore. 
-
 ### Supported algorithms
 
 See below for a table of supported algorithms. Algorithm identifiers match