Merge pull request torvalds#126 from ajeddeloh/update-readme

readme: update to include how to use this repo

Author: ajeddeloh

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
+```