kernel config project: add a writeup

Add a writeup of how the kernel config project designed to behave when
migrating kernel versions.

Signed-off-by: Tycho Andersen <tycho@docker.com>

projects/kernel-config/README.md

@@ -0,0 +1,38 @@
+## kernel config project
+
+The intent of the kernel config project is to demonstrate a better way to
+handle kernel config. Specifically:
+
+* support for arch and version specific config
+* make diffs as readable as possible
+* ensure that all of our config settings are kept after oldconfig
+
+We achieve the goals by:
+
+* having version-specific config in separate files, which are automatically
+  merged
+* only keeping track of visible symbols, only keeping track of a delta from
+  defconfig, and keeping symbols sorted alphabetically
+* checking after a `make oldconfig` in the kernel, that all of our symbols are
+  set as we want them to be
+
+The bulk of this work happens in makeconfig.sh, which merges the configs (and
+checks that the resulting config is okay).
+
+One important piece is generating a kernel config for a new version. There are
+a few cases:
+
+* A new kconfig symbol is introduced that we want to set a non-default value
+  of: in this case, we introduce a new `kernel_config.${VERSION}` file, and set
+  the value to what we want to set it to
+* A config symbol that was no-default before become the default: in this case,
+  we would move the non-default setting to version specific files for all of
+  the other versions, and not set anything for this new kernel, since what we
+  want is now the default.
+* A symbol we want to set is removed (or renamed), similar to the above, we
+  simply move the old symbol name to version specific files for older kernels
+  and put the new symbol name (if it exists) in the new version specific file
+
+When dropping support for an old kernel version, we just delete that version
+specific file, and promote any option that is present in all other versions to
+the common config file.