add README

README.md

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+# UTA tracker
+
+Tracking all the trains & buses in Salt Lake City.
+
+## API
+
+- `GET https://uta.clb.li/api.json`: List all vehicles currently in service (JSON)
+- `GET https://uta.clb.li/api`: Same as above, but Protobuf (see ![proto/schema.proto](./proto/schema.proto))
+- `GET https://uta.clb.li/api/shapes/{shape_id}`: Get the geographic route "shape" for a given vehicle's `shape_id`
+
+![screenshot](screenshot.png)