Add returnables to receipt case definitions for DE

doc/appendix-de-kassensichv/receipt-case-definitions/receipt-case-definitions.md

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+## Receipt Case Definitions
+
+This chapter expands on the definitions of Receipt Cases covered in Chapter ["Receipt Case Definitions"](../../general/receipt-case-definitions/receipt-case-definitions.md) of the General Part, with country specific information applicable to the German market.
+
+### Voucher (Service/Product)
+
+### Returnable
+
+Income from returnables (DE:Pfand) and their return with settlement of the pledged amount (DE: Pfandrückzahlung) are processes with DSFinV-K requirements. 
+
+#### Income from returnables (DE: Pfand)
+From a VAT point of view refering to the income from returnables (DE: Pfand), a distinction must be made here as to whether a transport container provided against a separately agreed deposit is a (self-contained) transport aid or merely a so-called goods container. Whereas transport aids basically serve to simplify the transport and storage of goods (e.g. pallets, boxes), goods are enclosed in inner/outer containers which are necessary for the delivery of the goods to the end consumer (e.g. bottles). 
+
+From the point of view of turnover tax, the provision of transport aids against a deposit represents an independent delivery which is subject to the general tax rate according to § 12 Abs. 1 UStG (currently 7% VAT). 
+
+In contrast, the encirclement of goods, as a so-called dependent secondary service, shares the fate of the actual main service/product. (e.g. delivery of milk 7% VAT - deposit on milk bottle also 7% VAT). 
+
+For transport aids please use the charge item case (`ftChargeItemCase`): `0x4445000000000022`. 
+
+For goods containers please use one of the following charge item cases depending on the VAT rate of the actual main service/product: `0x4445000000000021`, `0x4445000000000022`, `0x4445000000000023`, `0x4445000000000024`, `0x4445000000000025`, `0x4445000000000026` or `0x4445000000000027`.
+
+#### Return with settlement of the pledged amount (DE: Pfandrückzahlung)
+
+Analogue to the income from returnables. 
+
+For transport aids please use the charge item case (`ftChargeItemCase`): `0x444500000000002A` with negative amount.
+
+For goods containers please use one of the following charge item cases with negative amount depending on the VAT rate of the actual main service/product that was covered: `0x4445000000000029`, `0x444500000000002A`, `0x444500000000002B`, `0x444500000000002C`, `0x444500000000002D`, `0x444500000000002E` or `0x444500000000002F`.