Add discord shield into README.md (#46)

README.md

@@ -5,6 +5,7 @@
 
 [![CI][ci-img]][ci-url]
 [![Gitter chat][gitter-img]][gitter-url]
+[![Discord][discord-img]][discord-url]
 [![Coverage Status][coverage-img]][coverage-url]
 [![server pulls][docker-server-pulls-img]][docker-server-image-url]
 [![ui pulls][docker-ui-pulls-img]][docker-ui-image-url]
@@ -185,10 +186,10 @@ connection with your use of FFmpeg.
 
 ## Where to ask questions
 
-[Gitter chat](https://gitter.im/opencv-cvat): you can post CVAT usage related questions there.
+[Gitter chat][gitter-url]: you can post CVAT usage related questions there.
 Typically they get answered fast by the core team or community. There you can also browse other common questions.
 
-[Discord](https://discord.gg/fNR3eXfk6C) is the place to also ask questions or discuss any other stuff related to CVAT.
+[Discord][discord-url] is the place to also ask questions or discuss any other stuff related to CVAT.
 
 [GitHub issues](https://github.com/cvat-ai/cvat/issues): please post them for feature requests or bug reports.
 If it's a bug, please add the steps to reproduce it.
@@ -224,3 +225,6 @@ questions and get our support.
 
 [doi-img]: https://zenodo.org/badge/139156354.svg
 [doi-url]: https://zenodo.org/badge/latestdoi/139156354
+
+[discord-img]: https://img.shields.io/discord/1000789942802337834?label=discord
+[discord-url]: https://discord.gg/fNR3eXfk6C