Add conan badge

Author: MartinDelille

README.md

@@ -1,6 +1,7 @@
 # RtMidi
 
 ![Build Status](https://github.com/thestk/rtmidi/actions/workflows/ci.yml/badge.svg)
+[![Conan Center](https://shields.io/conan/v/rtmidi)](https://conan.io/center/rtmidi)
 
 A set of C++ classes that provide a common API for realtime MIDI input/output across Linux (ALSA & JACK), Macintosh OS X (CoreMIDI & JACK) and Windows (Multimedia).