docs: Add release notes for v1.8.1.

docs/release-notes/release-notes-1.8.1.md

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+# dcrd v1.8.1
+
+This is a patch release of dcrd that includes some updates to the RPC server and
+JSON-RPC API in light of the changes made by
+[DCP0011](https://github.com/decred/dcps/blob/master/dcp-0011/dcp-0011.mediawiki) as follows:
+
+* The `getblock` and `getblockheader` RPCs now have an additional `powhash`
+  field for the new Proof-of-Work hash
+* The `getnetworkhashps` RPC now treats -1 for the blocks parameter as the
+  default number of blocks versus the previous behavior that is no longer
+  applicable to the new difficulty adjustment algorithm
+
+The RPC server version as of this release is 8.1.0.
+
+## Changelog
+
+This patch release consists of 5 commits from 2 contributors which total to 7
+files changed, 47 additional lines of code, and 29 deleted lines of code.
+
+All commits since the last release may be viewed on GitHub
+[here](https://github.com/decred/dcrd/compare/release-v1.8.0...release-v1.8.1).
+
+### RPC:
+
+- rpc: Add PoWHash to getblock/getblockheader (verbose) results ([decred/dcrd#3192](https://github.com/decred/dcrd/pull/3192))
+- rpcserver: Modify getnetworkhashps -1 blocks logic ([decred/dcrd#3193](https://github.com/decred/dcrd/pull/3193))
+
+### Developer-related package and module changes:
+
+- jsonrpc/types: Add powhash to verbose block output ([decred/dcrd#3192](https://github.com/decred/dcrd/pull/3192))
+- main: Use backported rpc types updates ([decred/dcrd#3192](https://github.com/decred/dcrd/pull/3192))
+
+### Misc:
+
+- release: Bump for 1.8.1 ([decred/dcrd#3194](https://github.com/decred/dcrd/pull/3194))
+
+### Code Contributors (alphabetical order):
+
+- Dave Collins
+- Jonathan Chappelow