UPSTREAM.md: document danctnix codeberg submission route

Signed-off-by: Markus Fritsche <fritsche.markus@gmail.com>

UPSTREAM.md

@@ -160,6 +160,31 @@ open an MR against Mobian:
 4. Consider bundling into a single MR with all 6 commits if the Mobian
    reviewer prefers series review.
 
+### Near-term alt: danctnix linux-pinetab2 (codeberg)
+
+Danctnix is the de-facto upstream for PineTab2 Arch-ARM images
+(maintainer Danct12). The driver ships in-tree at
+`drivers/staging/bes2600/` in `codeberg.org/DanctNIX/linux-pinetab2`.
+
+A danctnix-layout variant of the series (same content, paths rooted at
+`drivers/staging/bes2600/` instead of Mobian's `bes2600/`) is at
+`marfrit/besser/patches/staging-prep-series-danctnix/`. Verified to
+apply cleanly against tag `v6.19.10-danctnix1` and checkpatch-clean.
+
+Submission route:
+1. Create a codeberg.org account + add your SSH key.
+2. Fork `DanctNIX/linux-pinetab2` on codeberg; push the 6 commits as a
+   branch `bes2600/staging-prep-series`.
+3. Open a codeberg PR against `DanctNIX/linux-pinetab2:master` with
+   the cover-letter content as PR description.
+4. Alt: ping Danct12 directly (Matrix / IRC `#pine64` / mastodon
+   `@danctnix@social.treehouse.systems`). Their merge workflow may
+   be more casual than formal PR review.
+
+Mobian and danctnix share the bes2600 source — `bes_pwr.c`,
+`bes2600_factory.c`, `wsm.c` all 0-hunk diff at the patched files.
+Landing in either benefits users of both.
+
 ### Longer-term: linux-wireless RFC for drivers/staging/bes2600/
 
 This series cleans up the Mobian DKMS tree. A proper mainline staging