docs: Add release notes for v2.0.4.

docs/release-notes/release-notes-2.0.4.md

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+# dcrd v2.0.4 Release Notes
+
+This is a patch release of dcrd which includes the following changes:
+
+- Improved session formation for StakeShuffle mix transactions
+- Support for Internationalized Domain Names (IDNs) in hostnames
+- StakeShuffle mixing performance enhancements
+
+## Changelog
+
+This patch release consists of 14 commits from 3 contributors which total to 17
+files changed, 201 additional lines of code, and 97 deleted lines of code.
+
+All commits since the last release may be viewed on GitHub
+[here](https://github.com/decred/dcrd/compare/release-v2.0.3...release-v2.0.4).
+
+### Mixing message relay (mix pool):
+
+- [release-v2.0] mixpool: Reject KEs submitted too early ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] mixclient: Use newest (fewest-PR) KEs to form alt sessions ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+
+### RPC / gencerts utility changes:
+
+- [release-v2.0] certgen,gencerts: Punycode non-ASCII hostnames ([decred/dcrd#3432](https://github.com/decred/dcrd/pull/3432))
+
+### Developer-related package and module changes:
+
+- [release-v2.0] mixclient: Remove completely unused var ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] mixpool: Remove error which is always returned nil ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] mixclient: Dont append to slice with non-zero length ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] mixing: Add missing copyright headers ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] mixclient: Add missing copyright headers ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] mixclient: Remove submit queue channel ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] mixclient: Do not submit PRs holding client mutex ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+
+### Developer-related module management:
+
+- [release-v2.0] main: Use backported mixing updates ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] main: Use backported certgen updates ([decred/dcrd#3432](https://github.com/decred/dcrd/pull/3432))
+
+### Misc:
+
+- [release-v2.0] mixing: Add missing periods to comments ([decred/dcrd#3431](https://github.com/decred/dcrd/pull/3431))
+- [release-v2.0] release: Bump for 2.0.4 ([decred/dcrd#3433](https://github.com/decred/dcrd/pull/3433))
+
+### Code Contributors (alphabetical order):
+
+- Dave Collins
+- Jamie Holdstock
+- Josh Rickmar