doc: Document how to enable qemu binfmt service and how to use it.

* doc/contributing.texi (Submitting Patches): Document how to enable qemu
binfmt service and how to use it.

Co-authored-by: Ricardo Wurmus <rekado@elephly.net>

doc/contributing.texi

@@ -817,6 +817,33 @@ name of the new or modified package, and fix any errors it reports
 Make sure the package builds on your platform, using @code{guix build
 @var{package}}.
 
+@item
+We recommend you also try building the package on other supported
+platforms.  As you may not have access to actual hardware platforms, we
+recommend using the @code{qemu-binfmt-service-type} to emulate them.  In
+order to enable it, add the following service to the list of services in
+your @code{operating-system} configuration:
+
+@example
+(service qemu-binfmt-service-type
+ (qemu-binfmt-configuration
+   (platforms (lookup-qemu-platforms "arm" "aarch64" "ppc" "mips64el"))
+   (guix-support? #t)))
+@end example
+
+Then reconfigure your system.
+
+You can then build packages for different platforms by specifying the
+@code{--system} option.  For example, to build the "hello" package for
+the armhf, aarch64, powerpc, or mips64 architectures, you would run the
+following commands, respectively:
+@example
+guix build --system=armhf-linux --rounds=2 hello
+guix build --system=aarch64-linux --rounds=2 hello
+guix build --system=powerpc-linux --rounds=2 hello
+guix build --system=mips64el-linux --rounds=2 hello
+@end example
+
 @item
 @cindex bundling
 Make sure the package does not use bundled copies of software already