docs: add docs for custom fields, rename template

README.MD

@@ -8,10 +8,18 @@ This Template is adopted from the original one with some minor changes:
 - Altered spacing and fields due to DIN 5008
 - Moved zip-code in front of city
 
+This Template has hardcoded German text.
+
 ## Installation
 
 Simply add the files into `application/views/invoice_templates/pdf` and select them in Invoiceplane's *System Settings -> Invoices* as PDF templates.
 
+### Extra fields
+
+You can add a "Leistungszeitraum" for any invoice by defining the two custom date fields: `Leistungszeitraum-Anfang` and `Leistungszeitraum-Ende`. These will not be shown on the invoice if either is not set.
+
+You can also add a description text for the full invoice by adding a text custom field called `Beschreibung`.
+
 ## Example
 
 ![Screenshot](screenshot.png)