Add release notes

RELEASE_NOTES.md

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+# Twirp-Ruby: Release notes
+
+This are the Ruby release notes, not the [Go toolchain ones](https://github.com/arthurnn/twirp-ruby/releases).
+
+## 1.12.0
+
+https://github.com/arthurnn/twirp-ruby/compare/v1.10.0...v1.12.0
+
+- [Expose response HTTP headers to the caller](https://github.com/arthurnn/twirp-ruby/pull/119)
+- [Allow google-protobuf v4](https://github.com/arthurnn/twirp-ruby/pull/122)
+- Lock Rack version to be < 3, until we fix Rack 3 compatibility
+
+## 1.11.0 - not released
+
+No ruby release for this version, despite an existent tag.
+https://github.com/arthurnn/twirp-ruby/compare/v1.10.0...v1.11.0
+
+There was no ruby change for this version therefor not release, instead that mark the de-coupling of the Go toolchain with the ruby version.
+
+## 1.10.0 and prior
+
+See the [releases](https://github.com/arthurnn/twirp-ruby/releases) page for the previous notes