Add discord label to README
[![Dependabot Status]][dependabot.com]
[![CII Best Practices]][bestpractices]
[![GitHub]](LICENSE)
+[![Discord]][discord]
[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
[CII Best Practices]: https://bestpractices.coreinfrastructure.org/projects/4356/badge
[bestpractices]: https://bestpractices.coreinfrastructure.org/projects/4356
[GitHub]: https://img.shields.io/github/license/tremor-rs/tremor-runtime
+[Discord]: https://img.shields.io/discord/752801695066488843.svg?label=&logo=discord&logoColor=ffffff&color=7389D8&labelColor=6A7EC2
+[discord]: https://bit.ly/tremor-discord
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