fix fiat conversion - requires cg api key

providers/bitcoin/README.md

@@ -5,4 +5,5 @@ A provider for bitcoin, backed by blockstream.info to query address balance and
 ### Environment Variables
 
 * *bitcoin_address_[0...]* - A series of bitcoin addresses with a suffix of _0 and each additional address counting up.
-* *bitcoin_ynab_account* - The YNAB account ID used to keep track of Bitcoin balance
+* *bitcoin_ynab_account* - The YNAB account ID used to keep track of Bitcoin balance
+* *bitcoin_coingecko_api_key* - Required for fiat conversion, your addresses nor balances are not transmitted. Docs: https://docs.coingecko.com/reference/setting-up-your-api-key