docs: expand macOS platform support coverage

We initially claimed to only support the most recent macOS
release, which is currently 10.15. Our Travis CI, however,
is validating 10.14.4 / XCode 10.3.

For almost all of our other platforms, we support multiple
releases to some degree. This change brings macOS in line
with other long life distros, covering the most recent &
most recent but one for a 2 year overlap. With this docs
change our CI is now actually testing our minimum version.
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

docs/platforms.html.in

@@ -74,8 +74,17 @@
     <h3>macOS</h3>
 
     <p>
-      The project supports building with the current version of macOS,
-      with the current homebrew package set available.
+      The project aims to support the most recent major version
+      at all times. Support for the previous major version will
+      be dropped 2 years after the new major version is released.
+    </p>
+
+    <p>
+      Note that to compile libvirt will require extra packages
+      to be made available on the macOS host. It is recommended
+      to use <a href="https://brew.sh/">HomeBrew</a> since this
+      is what libvirt CI tests with, however, <a herf="https://www.macports.org/">MacPorts</a>
+      is an alternative option that is likely to work.
     </p>
 
     <h3>FreeBSD</h3>