Add basic invoices setup

README.md

@@ -33,7 +33,11 @@ Then type code like:
 ```ruby
 account = StrikeMe::API::Account.new
 account.profile.attributes
-account.balances.map(&:attributes)
+account.balances.map(&:attributes) # same as account.balances.to_a
+account.balances.btc # to get the BTC specific one
+
+invoices = StrikeMe::API::Invoices.new
+invoices.any?
 ```
 
 Usage