Update readme

README.md

@@ -7,7 +7,7 @@ A platform where people can trade their plants. You can post what you have and s
 
 ## Tech stack
 - [Django](https://djangoproject.com/)
-- [MariaDB](https://www.postgresql.org/)
+- [Postgres](https://www.postgresql.org/)
 
 ## Admin dashboard
 Find it under `/admin`
@@ -31,4 +31,8 @@ Searching with filters such as:
 | Category     | `Category`    | dropdown   |
 
 ## Development
-To get started with development, see [DEVELOPMENT.md](DEVELOPMENT.md)
+To get started with development, see [DEVELOPMENT.md](DEVELOPMENT.md)
+
+## Open Source Data
+For calculating distances between zip codes, the `PLZ_Verzeichnis` is used.  
+Source: https://opendata.swiss/de/dataset/plz_verzeichnis