readme: update style

Update the style to one sentence per line, as is currently used in the FreeBSD
document project. Make the links to the handbook clickable.

README.md

 FreeBSD Source:
 ---------------
-This is the top level of the FreeBSD source directory.  This file
-was last revised on:
-$FreeBSD$
+This is the top level of the FreeBSD source directory.
 
-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.
+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.
+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.
+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), [FreeBSD handbook on building userland](https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/makeworld.html), and [Handbook for kernels](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
+| Directory | Description |
+| --------- | ------------ | 
+| 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](crypto/README)). |
+| etc | Template files for /etc. |
+| gnu | Various commands and libraries under the GNU Public License. Please see [gnu/COPYING](gnu/COPYING) and [gnu/COPYING.LIB](gnu/COPYING.LIB) 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](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 [FreeBSD Handbook](https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/current-stable.html).