readme.md: add link to libera.chat
@@ -7,6 +7,7 @@ SDR++ is a cross-platform and open source SDR software with the aim of being blo
* [Patreon](https://patreon.com/ryzerth)
* [Discord Server](https://discord.gg/aFgWjyD)
* IRC: `#sdrpp` ([libera.chat](https://libera.chat))
## Features
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