README: update content and format

* Change to markdown format
* Include an introduction
* Remove the commit mailing list from the contacts
* Add the IRC channel to the contacts
* Move 'S/Key' section to doc/README.skey
* Move authors and maintainers to AUTHORS.md

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

README.md

@@ -0,0 +1,36 @@
+# shadow-utils
+
+## Introduction
+The shadow-utils package includes the necessary programs for
+converting UNIX password files to the shadow password format, plus
+programs for managing user and group accounts. The pwconv command
+converts passwords to the shadow password format. The pwunconv command
+unconverts shadow passwords and generates a passwd file (a standard
+UNIX password file). The pwck command checks the integrity of password
+and shadow files. The lastlog command prints out the last login times
+for all users. The useradd, userdel, and usermod commands are used for
+managing user accounts. The groupadd, groupdel, and groupmod commands
+are used for managing group accounts.
+
+## Sites
+* [Homepage](https://github.com/shadow-maint/shadow)
+* [Issue tracker](https://github.com/shadow-maint/shadow/issues)
+* [Releases](https://github.com/shadow-maint/shadow/releases)
+
+## Contacts
+There are several ways to contact us:
+* [the general discussion mailing list](
+  https://alioth-lists.debian.net/mailman/listinfo/pkg-shadow-devel)
+* the #shadow IRC channel on libera.chat:
+  * irc://irc.libera.chat/shadow
+
+### Mailing archives
+* [the general discussion mailing list archive](
+  https://alioth-lists.debian.net/pipermail/pkg-shadow-devel/)
+* [the commit mailing list archive](
+  https://alioth-lists-archive.debian.net/pipermail/pkg-shadow-commits/),
+  only used for historical purposes
+
+## Authors and maintainers
+Authors and maintainers are listed in [AUTHORS.md](
+https://github.com/shadow-maint/shadow/blob/master/AUTHORS.md).