add generated command

Author: ushios

README.md

@@ -15,6 +15,17 @@ TODO: Add a description
 
 *Automatically generated by the [OpenAPI Generator](https://openapi-generator.tech)*
 
+Like below
+
+```
+openapi-generator-cli generate \
+  -i https://raw.githubusercontent.com/FIWARE/specifications/master/OpenAPI/ngsiv2/ngsiv2-openapi.json \
+  -g java \
+  --library restclient \
+  --additional-properties=invokerPackage=city.makeour.ngsi.v2.invoker,apiPackage=city.makeour.ngsi.v2.api,modelPackage=city.makeour.ngsi.v2.model,groupId=city.makeour.ngsi.v2,artifactId=ngsiv2,artifactVersion=1.0.0 \
+  -o ./ngsiv2-java
+```
+
 ## Requirements
 
 Building the API client library requires: