Add Github topics to repo

[emanuelb]

Improve project discoverability by adding github topics, see:
https://help.github.com/en/github/administering-a-repository/classifying-your-repository-with-topics

Related topics are:
https://github.com/topics/bitcoin-wallet
https://github.com/topics/bitcoin
https://github.com/topics/btc
https://github.com/topics/cryptocurrency
https://github.com/topics/blockchain-wallet
https://github.com/topics/wallet