README: Add linux to macOS build info

The README lists the URLs for the wiki pages describing
how to build on Linux and Windows; add the equivalent
link for building on macOS.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

README

@@ -45,6 +45,7 @@ of other UNIX targets. The simple steps to build QEMU are:
 Additional information can also be found online via the QEMU website:
 
   http://qemu-project.org/Hosts/Linux
+  http://qemu-project.org/Hosts/Mac
   http://qemu-project.org/Hosts/W32