doc: add historical release notes for 0.20.1

doc/release-notes/release-notes-0.20.1.md

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+0.20.1 Release Notes
+====================
+
+Bitcoin Core version 0.20.1 is now available from:
+
+  <https://bitcoincore.org/bin/bitcoin-core-0.20.1/>
+
+This minor release includes various bug fixes and performance
+improvements, as well as updated translations.
+
+Please report bugs using the issue tracker at GitHub:
+
+  <https://github.com/bitcoin/bitcoin/issues>
+
+To receive security and update notifications, please subscribe to:
+
+  <https://bitcoincore.org/en/list/announcements/join/>
+
+How to Upgrade
+==============
+
+If you are running an older version, shut it down. Wait until it has completely
+shut down (which might take a few minutes in some cases), then run the
+installer (on Windows) or just copy over `/Applications/Bitcoin-Qt` (on Mac)
+or `bitcoind`/`bitcoin-qt` (on Linux).
+
+Upgrading directly from a version of Bitcoin Core that has reached its EOL is
+possible, but it might take some time if the data directory needs to be migrated. Old
+wallet versions of Bitcoin Core are generally supported.
+
+Compatibility
+==============
+
+Bitcoin Core is supported and extensively tested on operating systems
+using the Linux kernel, macOS 10.12+, and Windows 7 and newer.  Bitcoin
+Core should also work on most other Unix-like systems but is not as
+frequently tested on them.  It is not recommended to use Bitcoin Core on
+unsupported systems.
+
+From Bitcoin Core 0.20.0 onwards, macOS versions earlier than 10.12 are no
+longer supported. Additionally, Bitcoin Core does not yet change appearance
+when macOS "dark mode" is activated.
+
+Known Bugs
+==========
+
+The process for generating the source code release ("tarball") has changed in an
+effort to make it more complete, however, there are a few regressions in
+this release:
+
+- The generated `configure` script is currently missing, and you will need to
+  install autotools and run `./autogen.sh` before you can run
+  `./configure`. This is the same as when checking out from git.
+
+- Instead of running `make` simply, you should instead run
+  `BITCOIN_GENBUILD_NO_GIT=1 make`.
+
+Notable changes
+===============
+
+Changes regarding misbehaving peers
+-----------------------------------
+
+Peers that misbehave (e.g. send us invalid blocks) are now referred to as
+discouraged nodes in log output, as they're not (and weren't) strictly banned:
+incoming connections are still allowed from them, but they're preferred for
+eviction.
+
+Furthermore, a few additional changes are introduced to how discouraged
+addresses are treated:
+
+- Discouraging an address does not time out automatically after 24 hours
+  (or the `-bantime` setting). Depending on traffic from other peers,
+  discouragement may time out at an indeterminate time.
+
+- Discouragement is not persisted over restarts.
+
+- There is no method to list discouraged addresses. They are not returned by
+  the `listbanned` RPC. That RPC also no longer reports the `ban_reason`
+  field, as `"manually added"` is the only remaining option.
+
+- Discouragement cannot be removed with the `setban remove` RPC command.
+  If you need to remove a discouragement, you can remove all discouragements by
+  stop-starting your node.
+
+Notification changes
+--------------------
+
+`-walletnotify` notifications are now sent for wallet transactions that are
+removed from the mempool because they conflict with a new block. These
+notifications were sent previously before the v0.19 release, but had been
+broken since that release (bug
+[#18325](https://github.com/bitcoin/bitcoin/issues/18325)).
+
+PSBT changes
+------------
+
+PSBTs will contain both the non-witness utxo and the witness utxo for segwit
+inputs in order to restore compatibility with wallet software that are now
+requiring the full previous transaction for segwit inputs. The witness utxo
+is still provided to maintain compatibility with software which relied on its
+existence to determine whether an input was segwit.
+
+0.20.1 change log
+=================
+
+### Mining
+- #19019 Fix GBT: Restore "!segwit" and "csv" to "rules" key (luke-jr)
+
+### P2P protocol and network code
+- #19219 Replace automatic bans with discouragement filter (sipa)
+
+### Wallet
+- #19300 Handle concurrent wallet loading (promag)
+- #18982 Minimal fix to restore conflicted transaction notifications (ryanofsky)
+
+### RPC and other APIs
+- #19524 Increment input value sum only once per UTXO in decodepsbt (fanquake)
+- #19517 psbt: Increment input value sum only once per UTXO in decodepsbt (achow101)
+- #19215 psbt: Include and allow both non_witness_utxo and witness_utxo for segwit inputs (achow101)
+
+### GUI
+- #19097 Add missing QPainterPath include (achow101)
+- #19059 update Qt base translations for macOS release (fanquake)
+
+### Build system
+- #19152 improve build OS configure output (skmcontrib)
+- #19536 qt, build: Fix QFileDialog for static builds (hebasto)
+
+### Tests and QA
+- #19444 Remove cached directories and associated script blocks from appveyor config (sipsorcery)
+- #18640 appveyor: Remove clcache (MarcoFalke)
+
+### Miscellaneous
+- #19194 util: Don't reference errno when pthread fails (miztake)
+- #18700 Fix locking on WSL using flock instead of fcntl (meshcollider)
+
+Credits
+=======
+
+Thanks to everyone who directly contributed to this release:
+
+- Aaron Clauson
+- Andrew Chow
+- fanquake
+- Hennadii Stepanov
+- João Barbosa
+- Luke Dashjr
+- MarcoFalke
+- MIZUTA Takeshi
+- Pieter Wuille
+- Russell Yanofsky
+- sachinkm77
+- Samuel Dobson
+- Wladimir J. van der Laan
+
+As well as to everyone that helped with translations on
+[Transifex](https://www.transifex.com/bitcoin/bitcoin/).