Add details ta Custom-Firmware.md (#503)

* Add details ta Custom-Firmware.md

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Custom-Firmware.md

@@ -6,6 +6,44 @@ https://github.com/rusefi/fw-custom-example is an example of a custom rusEFI fir
 
 https://github.com/rusefi/fw-custom-paralela and https://github.com/rusefi/fw-custom-core8/ are the best example as of right now. https://github.com/rusefi/rusefi/tree/master/firmware/config/boards should be another source of inspiration.
 
+If you have a more complex situation, with several different board variants you need to build, you can still achieve this with one repo.  
+If, for example, your variants only need different compile flags, having multiple meta-info files within the same repository would be a good way to achieve it.  
+For example:
+
+- meta-info-brainboard.env
+- meta-info-brainboard-debug.env
+
+Now, in `.github/workflows/build-firmware.yaml`, you need a [matrix](https://docs.github.com/en/actions/learn-github-actions/contexts#example-usage-of-the-matrix-context) to run the build firmware action for two different targets. It should look something like this:
+
+```yaml
+name: Create Board Firmware
+
+on:
+  push:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        bundle-name:
+        - brainboard
+        - brainboard-debug
+
+    steps:
+      call-workflow-passing-data:
+        uses: rusefi/rusefi/.github/workflows/custom-board-build.yaml@master
+        secrets: inherit
+        permissions:
+          contents: write
+        with:
+          bundle-name: ${{matrix.bundle-name}}
+```
+
+If you want to have several different boards in the same repo, you should put them in different sub-directories, then pass a `board-dir` to the workflow.
+
+There is also a `rusefi-dir` option you can use if you want to put your rusefi submodule somewhere other than `ext/rusefi`.
+
 ## Update rusEFI Reference
 
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