adding src/README.md

[coypu%sdf.org@localhost]

GitHub will display the README.md (markdown) or README (plaintext) of a
project's top-level. This has been copied by most competitors to GitHub
as well, and is seen as the standard way to provide a short introduction
to a project.

I created one, and I'd like to add it.
The biggest concern about it is being an additional source of
information that becomes out of date -- and I don't believe the
information will need much updating in the near future.

It uses markdown because that produces a friendlier result. I think the
text is just as readable when viewed as plaintext.

When viewed as markdown, it looks like this:
https://i.imgur.com/iVAXVKy.png

Let me know if you have any objections. I'd like to commit it soon.

NetBSD
------

NetBSD is a complete operating system.  
It supports a wide range of machines from recent arm64 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@localhost:/cvsroot checkout -P src

Additional documentation
------------------------

[The NetBSD Guide](https://www.netbsd.org/docs/guide/en/)