Add a README.md.

This document attempts to describe some commonly needed use-cases.
The build.sh command is opinionated but it's for the sake of having
a command that can be copied and probably works without needing to
inform the user about what various options do.

Discussed in tech-toolchain. I've added some links suggested by kamil
and switched to calling arm64 'aarch64' as requested by richard earnshaw.

README.md

@@ -0,0 +1,48 @@
+NetBSD
+------
+
+NetBSD is a complete operating system.  
+It supports a wide range of machines from recent aarch64 and amd64
+machines to VAX and M68k.
+
+Building
+--------
+
+Cross-building is possible from most UNIX-like operating systems.  
+To build for amd64 (x86_64), in the src directory:  
+    ./build.sh -U -u -j4 -m amd64 -O ~/obj release
+
+Additional build information available in the BUILDING file.
+
+Binaries
+--------
+
+- [Daily builds](https://nycdn.netbsd.org/pub/NetBSD-daily/HEAD/latest/)  
+- [Releases](https://cdn.netbsd.org/pub/NetBSD/)
+
+Testing
+-------
+
+On a running NetBSD system  
+    cd /usr/tests; atf-run |atf-report
+
+Troubleshooting
+---------------
+
+- Bugs and patches can be sent [via web form](https://www.netbsd.org/cgi-bin/sendpr.cgi?gndb=netbsd).  
+- Several [mailing lists](https://www.netbsd.org/mailinglists/) exist.  
+  The netbsd-users list is a good choice for many problems.  
+- A community IRC channel exist on [#netbsd @ freenode](https://webchat.freenode.net/?channels=#netbsd)
+
+Latest sources
+--------------
+
+To fetch the main CVS repository:  
+    cvs -d anoncvs@anoncvs.NetBSD.org:/cvsroot checkout -P src
+
+Additional Links
+----------------
+
+[The NetBSD Guide](https://www.netbsd.org/docs/guide/en/)
+[NetBSD manual pages](http://man.netbsd.org/)
+[NetBSD Cross-Reference](https://nxr.netbsd.org/)