Add release notes

RELEASE_NOTES.md

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+# Release Notes
+
+## v1.1.0
+
+- Added optional supoprt for "slow stations".
+
+  Trains that don't stop at a slow station are marked as "fast", showing a small
+  icon.
+
+  NB: To use this feature you need to run a the custom web server below to get
+  the calling stations.
+
+- Implemented custom web server.
+
+  This is so that we can reduce the JSON response message from RTT to only the
+  fields we're interested in, and to add additional calling-at stations that are
+  too memory-intensive to run on-device.
+
+- Fixed incorrect destination station for cancelled trains.
+
+## v1.0.0
+
+- Initial release