Update README.md

Author: hartig

README.md

@@ -1,6 +1,11 @@
 # pgschema-to-apischema
 The tool in this repository extends a schema for Property Graphs into a GraphQL schema for a GraphQL API, where [the Property Graph schema is assumed to be represented using the GraphQL Schema Definition Language (SDL)](http://blog.liu.se/olafhartig/documents/graphql-schemas-for-property-graphs/). The actual approach to extend the Property Graph schemas into GraphQL API schemas (as implemented by this tool) is documented in the [wiki of this repo](https://github.com/LiUGraphQL/pgschema-to-apischema/wiki).
 
+## Prerequisites
+```bash
+$ pip3 install graphql-core-next
+```
+
 ## Example
 ```bash
 $ python3 generator.py \