kernel: Add README for guidance on how to use the configure script

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

diff --git a/kernel/README.md b/kernel/README.md
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+# Kernel
+
+## Configuration
+
+### Layout
+
+The kernel configuration files are split into individual configuration files for
+each architecture. This way, common options can be changed globally and can be
+overwritten for certain architectures which gives us a more consistent configuration
+across all architectures.
+
+### How to update the configuration?
+
+For these steps, you will have to change into a build environment and change to
+the kernel directory:
+
+  # pakfire-builder shell kernel.nm
+  (pakfire-shell)> cd /usr/src/packages/kernel-x.y.z...
+
+The following commands are available:
+
+#### oldconfig
+
+You can run this when updating the kernel to a new release or after changing any
+options manually. All unset or updated options will be prompted for and all
+configuration files for all architectures will be updated.
+
+  (pakfire-shell)> scripts/configure oldconfig
+
+#### olddefconfig
+
+Like oldconfig, but automatically answers all options with the default value.
+
+  (pakfire-shell)> scripts/configure olddefconfig
+
+#### menuconfig
+
+If you want to edit the configuration using the kernel's config editor, you can
+do it for the main architecture (which is x86_64 right now). All other configuration
+files will be updated afterwards and potentially prompted for options that diverge
+for other architectures.
+
+  (pakfire-shell)> scripts/configure menuconfig
+
+#### listnewconfig
+
+This will list all unset options.
+
+  (pakfire-shell)> scripts/configure listnewconfig