Add a readme for the "files" dir

Signed-off-by: Johanan Liebermann <jliebermann@microsoft.com>

images/capi/packer/files/flatcar/README.md

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+# Flatcar-Related Build Files
+
+This directory contains files needed for building Flatcar Container Linux CAPI images.
+
+The following subdirectories exist:
+
+- `clc` - contains [Container Linux Config][1] files.
+- `ignition` - contains [Ignition][2] files generated from the CLC files in the `clc` directory.
+- `scripts` - contains scripts which are used by the various Flatcar builds.
+
+## Ignition Files
+
+Some Flatcar builds (e.g. QEMU) require Ignition files during OS installation. These files can be
+consumed directly from the `ignition` directory. Ignition files are generated from CLC files by the
+[Container Linux Config Transpiler][3].
+
+### Adding New Files
+
+To add a new Ignition file, do the following:
+
+1. Place a CLC YAML file with the desired config in `clc`.
+1. Add the name of the file without an extension to the `ignition_files` variable under the
+  `gen-ignition` target in the [Makefile](../../../Makefile). For example, for a CLC file named
+  `foo.yaml`, add `foo` to the Make target.
+1. Run `make gen-ignition` under `images/capi`. A new Ignition file is generated under `ignition`.
+1. Commit both the CLC file and the resulting Ignition file and open a PR to merge the changes.
+
+Once the changes are merged, the new Ignition file can be referenced in Flatcar builds and consumed
+as a raw file directly from GitHub.
+
+### Changing Existing Files
+
+To change an existing Ignition file, do the following:
+
+1. Edit the relevant CLC YAML file in `clc`.
+1. Run `make gen-ignition` under `images/capi`. The corresponding Ignition file is updated under
+  `ignition`.
+1. Commit the changes and open a PR to merge them.
+
+[1]: https://flatcar-linux.org/docs/latest/provisioning/cl-config/
+[2]: https://flatcar-linux.org/docs/latest/provisioning/ignition/
+[3]: https://flatcar-linux.org/docs/latest/provisioning/config-transpiler/