readme: update to include how to use this repo

It is unclear how this repo fits into the build process or what it is
for. Update the readme to clarify this.

Author: Andrew Jeddeloh

README.md

@@ -1 +1,13 @@
-This repository contains the CoreOS branch of Linux. A new branch is created for each kernel version the distribution is rebased onto. The currently shipped version is available in the [overlay repository]( https://github.com/coreos/coreos-overlay/tree/master/sys-kernel/coreos-sources/files).
+This repository contains the CoreOS branches of Linux. A new branch is created for each kernel version the distribution is rebased onto. The currently shipped version is available in the [overlay repository](https://github.com/coreos/coreos-overlay/tree/master/sys-kernel/coreos-sources/files).
+
+This repository is not used by the build process at all; it merely exists for generating patchsets that live in coreos-overlay. To add a patch, check out the branch you want to add a patch to (probably v${VERSION}-coreos), apply the patch, then run `git format-patch ${TAG}` where tag is the upstream kernel tag (no "-coreos"). This generates a set of patches that can be used by `./revbump.sh` script in the `coreos-sources` directory in the overlay.
+
+```sh
+git checkout v4.13.16-coreos
+
+<Apply and commit your patch>
+
+git format-patch v4.13.16
+# check that the patches were generated correctly
+ls *.patch
+```