Update README to include contribution guidelines (#317)

Added a contribution guideline section to the README.

Author: thomas-samoht

README.md

@@ -1,6 +1,8 @@
 # GFModules - Viewer Web
 
 This viewer application can be used to find patient data and also provides an address book to find organization information.
+This app is part of the 'Generieke Functies, lokalisatie en addressering' project of the Ministry of Health,
+Welfare and Sport of the Dutch government.
 
 ## Disclaimer
 
@@ -59,3 +61,15 @@ Make sure you build the frontend assets locally first:
 ```
 
 After this you can access the application at [http://localhost:8500](http://localhost:8500).
+
+## Contribution
+
+As stated in the [Disclaimer](#disclaimer) this project and all associated code serve solely as documentation and
+demonstration purposes to illustrate potential system communication patterns and architectures.
+
+For that reason we will only accept contributions that fit this goal. We do appreciate any effort from the
+community, but because our time is limited it is possible that your PR or issue is closed without a full justification.
+
+If you plan to make non-trivial changes, we recommend to open an issue beforehand where we can discuss your planned changes. This increases the chance that we might be able to use your contribution (or it avoids doing work if there are reasons why we wouldn't be able to use it).
+
+Note that all commits should be signed using a gpg key.