Create sys/README.md

Add this primarily to document the sys/ subdirectories of the source tree.

This is a straight copy from the contents of hier(7). Improvements will
follow in other changes.

Reviewed by:	imp, emaste
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D37132

Author: mhorne

README.md

@@ -32,7 +32,7 @@ Source Roadmap:
 | secure | Cryptographic libraries and commands. |
 | share | Shared resources. |
 | stand | Boot loader sources. |
-| sys | Kernel sources. |
+| sys | Kernel sources (see [sys/README.md](sys/README.md)). |
 | 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. |

sys/README.md

@@ -0,0 +1,34 @@
+FreeBSD Kernel Source:
+----------------------
+
+This directory contains the source files and build glue that make up the FreeBSD
+kernel and its modules, including both original and contributed software.
+
+Source Roadmap:
+---------------
+| Directory | Description |
+| --------- | ----------- |
+| amd64 | AMD64 architecture support |
+| arm | ARM architecture support |
+| arm64 | ARMv8 architecture support |
+| cam | `cam(4)` and `ctl(4)` |
+| cddl | CDDL-licensed optional sources, including ZFS and DTrace |
+| ddb | `ddb(4)` |
+| fs | most filesystems |
+| dev | device drivers |
+| geom | `geom(4)` |
+| i386 | i386 (32 bit) architecture support |
+| kern | main part of the kernel |
+| net80211 | `net80211(4)` |
+| netgraph | `netgraph(4)` |
+| netinet | `inet(4)` |
+| netinet6 | `inet6(4)` |
+| netipsec | `ipsec(4)` |
+| netpfil | `ipfw(4)` and `pf(4)` |
+| opencrypto | `crypto(7)` |
+| powerpc | PowerPC/POWER architecture support |
+| riscv | RISC-V architecture support |
+| security | `audit(4)` and `mac(4)` |
+| sys | kernel headers |
+| ufs | Unix File System |
+| x86 | code shared by AMD64 and i386 architectures |