Remove README in favor of README.md

Complete the transition to README.md I started 3 years ago. Remove the
now-redundant README file. It's currently just README.md w/o the light markup
and adds no real value. This also allows us to use additional MarkDown
markup as we see fit w/o worrying about keeping things in sync.

README

-This is the top level of the FreeBSD source directory.  This file
-was last revised on:
-$FreeBSD$
-
-FreeBSD is an operating system used to power modern servers,
-desktops, and embedded platforms. A large community has
-continually developed it for more than thirty years. Its
-advanced networking, security, and storage features have
-made FreeBSD the platform of choice for many of the
-busiest web sites and most pervasive embedded networking
-and storage devices.
-
-For copyright information, please see the file COPYRIGHT in this
-directory. Additional copyright information also exists for some
-sources in this tree - please see the specific source directories for
-more information.
-
-The Makefile in this directory supports a number of targets for
-building components (or all) of the FreeBSD source tree.  See build(7), config(8),
-https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/makeworld.html, and
-https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/kernelconfig.html
-for more information, including setting make(1) variables.
-
-Source Roadmap:
----------------
-
-bin		System/user commands.
-
-cddl		Various commands and libraries under the Common Development
-		and Distribution License.
-
-contrib		Packages contributed by 3rd parties.
-
-crypto		Cryptography stuff (see crypto/README).
-
-etc		Template files for /etc.
-
-gnu		Various commands and libraries under the GNU Public License.
-		Please see gnu/COPYING* for more information.
-
-include		System include files.
-
-kerberos5	Kerberos5 (Heimdal) package.
-
-lib		System libraries.
-
-libexec		System daemons.
-
-release		Release building Makefile & associated tools.
-
-rescue		Build system for statically linked /rescue utilities.
-
-sbin		System commands.
-
-secure		Cryptographic libraries and commands.
-
-share		Shared resources.
-
-stand		Boot loader sources.
-
-sys		Kernel sources.
-
-sys/<arch>/conf Kernel configuration files. GENERIC is the configuration
-		used in release builds. NOTES contains documentation of
-		all possible entries.
-
-tests		Regression tests which can be run by Kyua.  See tests/README
-		for additional information.
-
-tools		Utilities for regression testing and miscellaneous tasks.
-
-usr.bin		User commands.
-
-usr.sbin	System administration commands.
-
-
-For information on synchronizing your source tree with one or more of
-the FreeBSD Project's development branches, please see:
-
-  https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/current-stable.html