Document the fork: upstream base commit and Palladium specifics

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

FORK.md

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+# Fork notes
+
+`plm-electrumx` is a fork of [spesmilo/electrumx](https://github.com/spesmilo/electrumx) adapted to
+Palladium (PLM), the coin of [palladiumcore](https://github.com/palladium-coin/palladiumcore).
+
+## Starting point
+
+- Upstream commit: `280cb339aa158e099eb936033eecd15719770651`
+  ("Merge pull request #389 from SomberNight/202607_release_2_0", upstream release 2.0.0, 3 July 2026)
+
+Every commit up to and including that one is unmodified upstream history. All Palladium-specific
+changes are made in the commits after it:
+
+    git log 280cb33..main
+
+## Updating from upstream
+
+    git remote add upstream https://github.com/spesmilo/electrumx.git   # once
+    git config rerere.enabled true
+    git fetch upstream
+    git log main..upstream/master --oneline
+    git merge upstream/master
+
+Palladium changes are kept small and localized (mainly the `Palladium*` classes at the end of
+`src/electrumx/lib/coins.py`) to keep these merges easy.
+
+## Palladium specifics
+
+- Palladium Core is based on Bitcoin Core 0.19. It has no `getindexinfo`, `txospenderindex` or
+  `submitpackage`, so the daemon version/index checks are disabled for `COIN=Palladium`, and
+  `blockchain.outpoint.subscribe` and `blockchain.transaction.broadcast_package` return a daemon
+  error to clients.
+- The daemon must run with `txindex=1`.
+- Same genesis blocks as Bitcoin (mainnet, testnet3, regtest), 80-byte headers, sha256d PoW, segwit.
+- Parameters were taken from `src/chainparams.cpp` and `src/chainparamsbase.cpp` of palladiumcore.