add matrix badge

README.md

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 # c2bluetooth
 
 [![Dart](https://github.com/CrewLAB/c2bluetooth/actions/workflows/dart.yml/badge.svg)](https://github.com/CrewLAB/c2bluetooth/actions/workflows/dart.yml)
+[![Matrix](https://img.shields.io/matrix/c2bluetooth:matrix.org?label=chat%20on%20matrix)](https://matrix.to/#/#c2bluetooth:matrix.org)
 
 C2Bluetooth is a flutter package designed to provide an easy API for accessing data from Concept2 PM5 Indoor fitness machines via bluetooth. This library implements the [Concept2 Performance Monitor Bluetooth Smart Communications Interface Definition](https://www.concept2.com/files/pdf/us/monitors/PM5_BluetoothSmartInterfaceDefinition.pdf) Specification ([newer versions](https://www.c2forum.com/viewtopic.php?f=15&t=193697#p527068) are also available). It also relies heavily on the [CSAFE specification](https://web.archive.org/web/20060718175014/http://www.fitlinxx.com/csafe/specification.htm) from FitLinxx.