sys/README.md: source roadmap improvements

Tweak the existing descriptions slightly. Add entries for several
directories which are not yet documented, but not exhaustively.

Reviewed by:	imp (previous version), emaste
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D37133

Author: mhorne

sys/README.md

@@ -8,27 +8,37 @@ 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 |
+| amd64 | AMD64 (64-bit x86) architecture support |
+| arm | 32-bit ARM architecture support |
+| arm64 | 64-bit ARM (AArch64) architecture support |
+| cam | Common Access Method storage subsystem - `cam(4)` and `ctl(4)` |
+| cddl | CDDL-licensed optional sources such as DTrace |
+| conf | kernel build glue |
+| compat | Linux compatibility layer, FreeBSD 32-bit compatibility |
+| contrib | 3rd-party imported software such as OpenZFS |
+| crypto | crypto drivers |
+| ddb | interactive kernel debugger - `ddb(4)` |
+| fs | most filesystems, excluding UFS, NFS, and ZFS |
 | dev | device drivers |
-| geom | `geom(4)` |
-| i386 | i386 (32 bit) architecture support |
+| gdb | kernel remote GDB stub - `gdb(4)` |
+| geom | GEOM framework - `geom(4)` |
+| i386 | i386 (32-bit x86) 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)` |
+| libkern | libc-like and other support functions for kernel use |
+| modules | kernel module infrastructure |
+| net | core networking code |
+| net80211 | wireless networking (IEEE 802.11) - `net80211(4)` |
+| netgraph | graph-based networking subsystem - `netgraph(4)` |
+| netinet | IPv4 protocol implementation - `inet(4)` |
+| netinet6 | IPv6 protocol implementation - `inet6(4)` |
+| netipsec | IPsec protocol implementation - `ipsec(4)` |
+| netpfil | packet filters - `ipfw(4)`, `pf(4)`, and `ipfilter(4)` |
+| opencrypto | OpenCrypto framework - `crypto(7)` |
+| powerpc | PowerPC/POWER (32 and 64-bit) architecture support |
+| riscv | 64-bit RISC-V architecture support |
+| security | security facilities - `audit(4)` and `mac(4)` |
 | sys | kernel headers |
-| ufs | Unix File System |
+| tests | kernel unit tests |
+| ufs | Unix File System - `ffs(7)` |
+| vm | virtual memory system |
 | x86 | code shared by AMD64 and i386 architectures |