README: update with instructions for external cpio/initramfs

Use the new meta-bsp-google-gs layer to create alternative initramfs
cpio images.

README.md

@@ -82,6 +82,32 @@ and is unmaintained at this stage:
 buildp21upstream.sh
 ```
 
+### Initramfs notes
+
+The build relies on an initramfs to boot the device. The default is to use the
+minimal busyboxx-based initramfs that comes with the repo clone.
+
+An alternative is to build an cpio-initramfs externally, and just point the
+Makefile at it, e.g. using yocto:
+
+```
+make PREBUILT_CPIO=/yocto-builds/yocto/build-google-gs-poky/tmp/deploy/images/google-gs/core-image-minimal-google-gs.cpio boot
+PREBUILT_CPIO=/yocto-builds/yocto/build-google-gs-poky/tmp/deploy/images/google-gs/core-image-full-cmdline.cpio make boot
+```
+
+The above can be generated using the yocto build from:
+
+```
+repo init -u https://git.codelinaro.org/linaro/googlelt/manifest.git -b oe/master -m default.xml
+repo sync -j64 --jobs-checkout=$(nproc) -c --no-tags
+MACHINE=google-gs DISTRO=poky EXTRA_LAYERS="meta-openembedded/meta-oe" . ./setup-environment /yocto-builds/yocto/build-google-gs-poky
+echo 'IMAGE_CLASSES:append:gs = " image-as-nokernel-initramfsrootfs"' >> conf/local.conf
+bitbake core-image-full-cmdline
+```
+
+Please see the yocto documentation for how to add additional packages to that
+image if desired.
+
 ## Example UART log
 
 ```