Add Gitpod badge to README
[![CII Best Practices]][bestpractices]
[![GitHub]](LICENSE)
[![Discord]][discord-invite]
+[![Gitpod][gitpod-hook]
[build status]: https://github.com/tremor-rs/tremor-runtime/workflows/Tests/badge.svg
[actions-tests]: https://github.com/tremor-rs/tremor-runtime/actions?query=workflow%3ATests
[discord-invite]: https://bit.ly/tremor-discord
[arm builds]: https://cloud.drone.io/api/badges/tremor-rs/tremor-runtime/status.svg
[drone.io]: https://cloud.drone.io/tremor-rs/tremor-runtime
+[gitpod]: https://gitpod.io/button/open-in-gitpod.svg
+[gitpod-hook]: https://gitpod.io/#https://github.com/tremor-rs/tremor-runtime
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