Update readme for mvn install

[jamesg1]

Thanks for putting together this repo, I'm new to AEM and seeing how React can work with it :)

I was getting some issues with maven dependencies when trying to install the project at react/chapter-4.

I had to add -Padobe-public to my mvn install's so adobe packages get installed or your settings.xml for m2 needs to get the repositories added - see https://helpx.adobe.com/experience-manager/using/sling_servlet_mbean.html#h2SetupMaveninyourdevelopmentenvironmenth2

Mentioning this in the readme would be good.

Ref: adobe/aem-project-archetype#94 (comment)