Mention ViaVersion addon in "Known issues" (#623)

README.md

@@ -34,7 +34,8 @@ Known issues
   1.17 will not be able to see or interact with blocks below y=0 and above y=255
 * <1.17 clients on 1.17+ servers might experience inventory desyncs on certain inventory click actions
 * Sound mappings are incomplete ([see here](https://github.com/ViaVersion/ViaBackwards/issues/326))
-* <1.19.4 clients on 1.20+ servers won't be able to use the smithing table
+* <1.19.4 clients on 1.20+ servers won't be able to use the smithing table, this can be fixed by
+  installing [AxSmithing](https://github.com/ViaVersionAddons/AxSmithing)
 
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