Add C++ reference

Author: sipa

bip-0173.mediawiki

@@ -247,6 +247,7 @@ P2PKH addresses can be used.
 
 * Reference encoder and decoder:
 ** [https://github.com/sipa/bech32/tree/master/ref/c For C]
+** [https://github.com/sipa/bech32/tree/master/ref/c++ For C++]
 ** [https://github.com/sipa/bech32/tree/master/ref/javascript For JavaScript]
 ** [https://github.com/sipa/bech32/tree/master/ref/go For Go]
 ** [https://github.com/sipa/bech32/tree/master/ref/python For Python]