doc: add reminder to update jhipster-online
doc: add reminder to update jhipster-online

This adds a small reminder to update jhipster-online when submitting a pull request.

https://github.com/jhipster/jhipster-online/pull/229#issuecomment-687653077
pascalgrimaud/generator-jhipster,ruddell/generator-jhipster,jhipster/generator-jhipster,jhipster/generator-jhipster,ruddell/generator-jhipster,ctamisier/generator-jhipster,atomfrede/generator-jhipster,ctamisier/generator-jhipster,atomfrede/generator-jhipster,liseri/generator-jhipster,mraible/generator-jhipster,mraible/generator-jhipster,PierreBesson/generator-jhipster,vivekmore/generator-jhipster,gmarziou/generator-jhipster,PierreBesson/generator-jhipster,dynamicguy/generator-jhipster,ruddell/generator-jhipster,atomfrede/generator-jhipster,dynamicguy/generator-jhipster,jhipster/generator-jhipster,vivekmore/generator-jhipster,vivekmore/generator-jhipster,ctamisier/generator-jhipster,dynamicguy/generator-jhipster,dynamicguy/generator-jhipster,PierreBesson/generator-jhipster,gmarziou/generator-jhipster,PierreBesson/generator-jhipster,jhipster/generator-jhipster,ctamisier/generator-jhipster,atomfrede/generator-jhipster,pascalgrimaud/generator-jhipster,ruddell/generator-jhipster,jhipster/generator-jhipster,ruddell/generator-jhipster,vivekmore/generator-jhipster,pascalgrimaud/generator-jhipster,gmarziou/generator-jhipster,ctamisier/generator-jhipster,mraible/generator-jhipster,liseri/generator-jhipster,pascalgrimaud/generator-jhipster,liseri/generator-jhipster,vivekmore/generator-jhipster,pascalgrimaud/generator-jhipster,liseri/generator-jhipster,gmarziou/generator-jhipster,liseri/generator-jhipster,mraible/generator-jhipster,atomfrede/generator-jhipster,gmarziou/generator-jhipster,PierreBesson/generator-jhipster,mraible/generator-jhipster