ts-web/signer-tweetnacl: add changelogs

client-sdk/ts-web/signer-tweetnacl/docs/changelog.md

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+# Changelog
+
+## Unreleased changes
+
+New features:
+
+- This is the `NaclSigner` class formerly in `@oasisprotocol/client`. Feel
+  free to continue using it for development.
+
+Breaking changes:
+
+- The `note` parameter is removed. Our opinion on using in-application-memory
+  keys is unchanged. But if you're taking the step of installing this library
+  called "signer-tweetnacl," you that it's using tweetnacl under the hood.