Add documentation regarding persisting FinTS

Also provide the key in the configuration example.json

Refs bnw#149

Readme.md

@@ -53,6 +53,8 @@ Instead of entering all necessary account information every time, you can load i
 Simply create such a JSON-file in the `data/configurations` folder by adapting the provieded [`data/configurations/example.json`](data/configurations/example.json). When starting the app in your browser, you can then choose the JSON-file as a configuration source.  
 Please note that the `bank_2fa`-value in the JSON file corresponds to the number of the 2-factor authentication as listed in [`app/public/html/collecting-data.twig`](app/public/html/collecting-data.twig).  
 Thanks to [joBr99](https://github.com/joBr99) for this feature!
+If you need to enter a TAN on every import run, you can paste the persistence string you get presented after the import into the configuration file.
+Treat this string as a secret, as it provides access to your bank account without a TAN!
 
 
 Headless usage

data/configurations/example.json

@@ -5,6 +5,7 @@
   "bank_url": "https://hbci11.fiducia.de/cgi-bin/hbciservlet",
   "bank_2fa": "944",
   "bank_2fa_device": "",
+  "bank_fints_persistence": "",
   "firefly_url": "",
   "firefly_access_token": "",
   "skip_transaction_review": "false",