README.md: Update "Make your own flavour" instructions
README.md: Update "Make your own flavour" instructions

vector-im/vector-android,vector-im/riot-android,vector-im/vector-android,vector-im/vector-android,vector-im/riot-android,vector-im/vector-android,vector-im/riot-android,vector-im/riot-android,vector-im/riot-android