sys-boot/shim: Add the README for the shim repo

Signed-off-by: Sayan Chowdhury <[email protected]>

sdk_container/src/third_party/coreos-overlay/sys-boot/shim/README.md

@@ -0,0 +1,12 @@
+shim
+====
+
+The repo is used to build the shim required for secure boot. The `flatcar/shim-review`
+repo hosts a `Dockerfile` that builds the shim ebuild and produces the binary
+required for shim-review. The generated `shim.efi` is then submitted for review.
+
+Once the signed shim is received, a release is cut in the `flatcar/shim-review`
+repo, which is then used during the build process. It's important to note that
+the version of the shim and the shim-signed ebuild should be the same. For
+example, if the current version of the shim is `15.8`, the ebuild files should
+be `shim-15.8.ebuild` and `shim-signed-15.8.ebuild` respectively.