add v0.12.1 release notes

doc/release-notes/release-notes-v0.12.1.md

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+### MMGen Version 0.12.1 Release Notes
+
+In addition to some new features, this release includes many changes “under the
+hood” that make the MMGen code base harder, more modular, and more extensible.
+
+#### Significant new features:
+
+ - asyncio/aiohttp support: f9a483f3
+ - txcreate,twview,listaddresses: display exact transaction date: b671453c
+ - mnemonic entry modes: 04add0df
+
+#### Significant code changes:
+
+ - data objects: always raise exception on failure: 0852321c
+ - new Lockable class: lock global vars after initialization: 4c2410e0
+ - eliminate global vars g.proto, g.coin, g.rpc and others: c3f185e8
+ - rewritten transaction classes: c3f185e8
+ - new LEDControl class: 5ba2f51e
+ - rewritten terminal code: d8e1d5f8
+
+This release has been tested on the following platforms:
+
+        Debian Buster / x86_64
+        Ubuntu Focal / x86_64
+        Armbian Bionic / Rock Pi 4 (armv8)
+        Armbian Bionic / Orange Pi PC2 (armv8)
+        Raspbian Buster / Raspberry Pi B (armv7) (BTC only)
+        Windows 10 Enterprise Eng. / MSYS2 / qemu-x86_64
+
+and with the following coin daemon versions:
+
+        Bitcoin Core 0.20.0
+        Bitcoin-ABC 0.21.8
+        Litecoin Core 0.18.1
+        Monerod 0.16.0.0
+        OpenEthereum 3.0.1
+
+Python version 3.6 or greater is required.
+
+Altcoin address generation has been additionally tested using the following
+tools as references:
+
+        zcash-mini a2b35042 (https://github.com/FiloSottile/zcash-mini)
+        pycoin 11f60a7c (https://github.com/richardkiss/pycoin)
+        vanitygen-plus 22123128 (https://github.com/exploitagency/vanitygen-plus)
+        MoneroPy 98e7feb2 (https://github.com/bigreddmachine/MoneroPy)
+        ethkey 2.7.2 (https://github.com/paritytech/parity-ethereum)