README notes re. grammarURI change needed in *.mwe2

This is something I stumbled across, and took me a minute to figure out while I was applying this maven-xtext-example to my https://github.com/vorburger/eclipse-typescript-xtext... may be a few words of doc about this are helpful to others in the future?

Depending on whether the New Project wizard in Xtext v2.5 has already been adapted to do this this, the wording of the doc should may be changed, and/or this is something that could be adapted as part of https://bugs.eclipse.org/bugs/show_bug.cgi?id=422375 in the future some day?

PS: I've noticed that there were some other differences in the GenerateHeroLanguage.mwe2 as well, which I didn't have in a 2.4 Wizard-based project (+resourceSet, +uriMap, -scanClassPath, +forcedResourceSet = theResourceSet) - no idea if those are relevant in this context?