[ci] Move cross-arch testing instructions to CONTRIBUTING.md

Signed-off-by: Tiyash Basu <[email protected]>

Author: tiyash-basu-frequenz

.github/containers/nox-cross-arch/README.md

@@ -115,10 +115,6 @@ That said, if you want to test the actual website in **your fork**, you can
 always use `mike deploy --push --remote your-fork-remote`, and then access the
 GitHub pages produced for your fork.
 
-##  Cross-Arch Testing
-
-This project has built-in support for testing across multiple architectures. Currently, our CI conducts tests on `aarch64` machines using QEMU emulation. We also have the flexibility to expand this support to include additional architectures in the future. For more information, see [Cross-Arch Testing](.github/containers/nox-cross-arch/README.md).
-
 ## Releasing
 
 These are the steps to create a new release:
@@ -158,3 +154,27 @@ These are the steps to create a new release:
    eventually too).
 
 7. Celebrate!
+
+##  Cross-Arch Testing
+
+This project has built-in support for testing across multiple architectures. Currently, our CI conducts tests on `arm64` machines using QEMU emulation. We also have the flexibility to expand this support to include additional architectures in the future.
+
+This project containers dockerfiles that can be used in the CI to test the
+python package in non-native machine architectures, e.g., `arm64`.
+The dockerfiles exist in the directory `.github/containers/nox-cross-arch`, and
+follow a naming scheme so that they can be easily used in build matrices in the
+CI, in `nox-cross-arch` job. The naming scheme is:
+
+```
+<arch>-<os>-python-<python-version>.Dockerfile
+```
+
+E.g.,
+
+```
+arm64-ubuntu-20.04-python-3.11.Dockerfile
+```
+
+If a dockerfile for your desired target architecture, OS, and python version
+does not exist here, please add one before proceeding to add your options to the
+test matrix.