Clarify requirements and process to get donation receipts

The process through Zwiebelfreunde is quite complex and described on their website in German:

It would be good to add this to our donation process in English and translated.

We have notes about this in the accounting repo: administrativa/Zwiebelfreunde/donation_receipts_info.mdwn.

Could be a goal to integrate this before the donation campaign in 2017 :)

Original created by @sajolida on 12036 (Redmine)

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