docs: add high-level overlay development guide

This provides rough step-by-step instructions on developing
an overlay and using the imager. It also provides some basic
information about different file types and their purpose.

Signed-off-by: Nicklas Frahm <nicklas.frahm@gmail.com>
Signed-off-by: Noel Georgi <git@frezbo.dev>

website/content/v1.8/advanced/overlays.md

@@ -43,3 +43,71 @@ An Overlay is a container image with the [specific folder structure](https://git
 Overlays can be built and managed using any tool that produces container images, e.g. `docker build`.
 
 Sidero Labs maintains a [repository of overlays](https://github.com/siderolabs/overlays).
+
+### Developing An Overlay
+
+Let's assume that you would like to contribute an overlay for a specific board, e.g. by contributing to the [`sbc-rockchip` repository](https://github.com/siderolabs/sbc-rockchip).
+Clone the repositry and insepct the existing overlays to understand the structure.
+
+Usually an overlay consist of a few key components:
+
+- `firmware`: contains the firmware files required for the board
+- `bootloader`: contains the bootloader, e.g. `u-boot` for the board
+- `dtb`: contains the device tree blobs for the board
+- `installer`: contains the installer that will be used to install this overlay on the node
+- `profile`: contains information
+
+1. For the new overlay, create any needed folders and `pkg.yaml` files.
+2. If your board introduces a new chipset that is not supported yet, make sure to add the firmware build for it.
+3. Add the necessary `u-boot` and `dtb` build steps to the `pkg.yaml` files.
+4. Proceed to add an installer, which is a small go binary that will be used to install the overlay on the node.
+    Here you need to add the go `src/` as well as the `pkg.yaml` file.
+5. Lastly, add the profile information in the `profiles` folder.
+
+You are now ready to attempt building the overlay.
+It's recommend to push the build to a container registry to test the overlay with the Talos installer.
+
+The default settings are:
+
+- `REGISTRY` is set to `ghcr.io`
+- `USERNAME` is set to the `siderolabs` (or value of environment variable `USERNAME` if it is set)
+
+```bash
+make sbc-rockchip PUSH=true
+```
+
+If using a custom registry, the `REGISTRY` and `USERNAME` variables can be set:
+
+```bash
+make sbc-rockchip PUSH=true REGISTRY=<registry> USERNAME=<username>
+```
+
+After building the overlay, take note of the pushed image tag, e.g. `664638a`, because you will need it for the next step.
+You can now build a flashable image using the command below.
+
+```bash
+export TALOS_VERSION=v1.7.6
+export USERNAME=octocat
+export BOARD=nanopi-r5s
+export TAG=664638a
+
+docker run --rm -t -v ./_out:/out -v /dev:/dev --privileged ghcr.io/siderolabs/imager:${TALOS_VERSION} \
+    metal --arch arm64 \
+    --base-installer-image="ghcr.io/siderolabs/installer:${TALOS_VERSION}" \
+    --overlay-name="${BOARD}" \
+    --overlay-image="ghcr.io/${USERNAME}/sbc-rockchip:${TAG}" \
+```
+
+> **--overlay-option**
+
+ `--overlay-option` can be used to pass additional options to the overlay installer if they are implemented by the overlay.
+ An example can be seen in the [sbc-raspberrypi](https://github.com/siderolabs/sbc-raspberrypi/) overlay repository.
+ It supports passing multiple options by repeating the flag or can be read from a yaml document by passing `--overlay-option=@<path to file>`.
+
+> **Note:** In some cases you need to build a custom imager.
+> In this case, refer to [this guide on how to build a custom images]({{< relref "./building-images" >}}) using an imager.
+
+#### Troubleshooting
+
+> **IMPORTANT:** If this does not succeed, have a look at the documentation of the external dependecies you are pulling in and make sure that the `pkg.yaml` files are correctly configured.
+> In some cases it may be required to update the dependencies to an appropriate version via the `Pkgfile`.