doc: Add a README file for NetBSD.

[skip ci]

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+# Compiling tcpdump on NetBSD
+
+NetBSD has two libpcap libraries: one that is always installed as a part of the
+OS and another that can be installed as a package from pkgsrc.  Also the usual
+method of compiling with the upstream libpcap in `../libpcap` is available.
+
+GCC, Clang, Autoconf and CMake are presumed to work, if this is not the case,
+please report a bug as explained in the
+[guidelines for contributing](../CONTRIBUTING.md).
+
+## NetBSD 9.3
+
+* Upstream libpcap works.
+* OS libpcap works.
+* pkgsrc libpcap 1.10.1 works only with Autoconf.
+
+## NetBSD 9.2, 9.1 and 9.0
+
+* Upstream libpcap works.
+* OS libpcap cannot be used due to
+  [this bug](https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=55901).
+