docs: Add release notes for v2.0.6.

docs/release-notes/release-notes-2.0.6.md

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+# dcrd v2.0.6 Release Notes
+
+This is a patch release of dcrd which includes the following changes:
+
+- Improved reliability for wallets participating in StakeShuffle mixing
+- Minor performance enhancement for random number generation on OpenBSD
+
+## Changelog
+
+This patch release consists of 3 commits from 2 contributors which total to 8
+files changed, 119 additional lines of code, and 68 deleted lines of code.
+
+All commits since the last release may be viewed on GitHub
+[here](https://github.com/decred/dcrd/compare/release-v2.0.5...release-v2.0.6).
+
+### Developer-related package and module changes:
+
+- [release-v2.0] rand: Use stdlib crypto/rand.Read on OpenBSD + Go 1.24 ([decred/dcrd#3473](https://github.com/decred/dcrd/pull/3473))
+
+### Developer-related module management:
+
+- [release-v2.0] main: Use backported rand updates ([decred/dcrd#3473](https://github.com/decred/dcrd/pull/3473))
+
+### Misc:
+
+- [release-v2.0] release: Bump for 2.0.6 ([decred/dcrd#3474](https://github.com/decred/dcrd/pull/3474))
+
+### Code Contributors (alphabetical order):
+
+- Dave Collins
+- Josh Rickmar