doc: Clean out release notes

0.12.0 was released, prepare empty release notes for 0.12.1.

doc/release-notes.md

@@ -16,6 +16,60 @@ Dash was previously known as Darkcoin.
 Darkcoin tree 0.8.x was a fork of Litecoin tree 0.8, original name was XCoin
 which was first released on Jan/18/2014.
 
+### Downgrade to a version < 0.12.0
+
+Because release 0.12.0 and later will obfuscate the chainstate on every
+fresh sync or reindex, the chainstate is not backwards-compatible with
+pre-0.12 versions of Bitcoin Core or other software.
+
+If you want to downgrade after you have done a reindex with 0.12.0 or later,
+you will need to reindex when you first start Bitcoin Core version 0.11 or
+earlier.
+
+Notable changes
+===============
+
+Example item
+---------------------------------------
+
+Example text.
+
+0.12.1 Change log
+=================
+
+Detailed release notes follow. This overview includes changes that affect
+behavior, not code moves, refactors and string updates. For convenience in locating
+the code changes and accompanying discussion, both the pull request and
+git merge commit are mentioned.
+
+### RPC and REST
+
+### Configuration and command-line options
+
+### Block and transaction handling
+
+### P2P protocol and network code
+
+### Validation
+
+### Build system
+
+### Wallet
+
+### GUI
+
+### Tests and QA
+
+### Miscellaneous
+
+Credits
+=======
+
+Thanks to everyone who directly contributed to this release:
+
+
+As well as everyone that helped translating on [Transifex](https://www.transifex.com/projects/p/bitcoin/).
+
 Darkcoin tree 0.9.x was the open source implementation of masternodes based on
 the 0.8.x tree and was first released on Mar/13/2014.