Add a note about preferred branch name for package imported from Debian Sid/Unstable

This is for consistency across all Apertis packages.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

content/guides/component_guide.md

@@ -103,6 +103,12 @@ have access to the Apertis Gitlab server, see
     apllying the default settings and triggering the CI pipeline on all
     downstream branches.
 
+  {{% notice info %}}
+  For consistency across all Apertis packages, when a package is imported from
+  Debian Sid/Unstable, the branch with the codename `sid` should be used:
+  `debian/sid` and not `debian/unstable`.
+  {{% /notice %}}
+
   {{% notice warning %}}
   Don't use `import-debian-package` on existing repositories, it does not
   attempt to merge `apertis/*` branches and instead it re-sets them to new