update README.md with cross compilation information

Fixes #1649

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>

README.md

@@ -173,6 +173,28 @@ qemu-system-x86_64 -kernel $KERNEL -initramfs /tmp/initramfs.linux_amd64.cpio -n
 (It fails to do that because some initialization is missing when the shell is
 started without a proper init.)
 
+## Cross Compilation (targeting different architectures and OSes)
+
+Cross-OS and -architecture compilation comes for free with Go. In fact, every PR
+to the u-root repo is built against the following architectures: amd64, x86
+(i.e. 32bit), mipsle, armv7, arm64, and ppc64le.
+
+Further, we run integration tests on linux/amd64, freebsd/amd64 and linux/arm64,
+using several CI systems. If you need to add another CI system, processor or OS,
+please let us know.
+
+To cross compile for an ARM, on Linux:
+
+```shell
+GOARCH=arm u-root
+```
+
+If you are on OSX, and wish to build for Linux on AMD64:
+
+```shell
+GOOS=linux GOARCH=amd64 u-root
+```
+
 ## Testing in QEMU
 
 A good way to test the initramfs generated by u-root is with qemu: