Support ING Wertpapiere as source

[Maxr1998]

Wrote myself a script to pull stock data from ING (my broker), might be useful for others as well.

[faiteanu]

Thanks a lot for your contribution. Would you mind updating docs/index.md as well with a link to the new file and a few words?

You might also want to include some advice on error conditions. For instance, you hardcoded time ranges and trade exchanges, which might not be valid for every stock, e.g. DE000A27Z304 is not tradable in USA (Nasdaq).
If you have a look at the other scripts, they actually parse the respective web page to determine the valid exchanges for each stock.