Add limiting scope to pom.xml as suggested in README

[DarrienG]

As the README states:

Derive4J should be declared as a compile-time only
dependency (not needed at runtime). So while derive4j is
(L)GPL-licensed, the generated code is not linked to derive4j, and thus
derive4j can be used in any project (proprietary or not).

However, the provided maven tags do not impose this requirement
potentially causing legal issues for folks who accidentally forget the
proper scope in proprietary codebases.

This PR ensures that the default pom example uses the recommended
practices.

This also removes a bit of unnecessary whitespace in the README.

[jbgi]

Yeah maven does not have a scope for compile-only dependencies.
I thought <optional>true</optional> was not enough to not be include derive4j in ear or war. its also excluded from transitive dependencies. Is there cases not covered by optional that are covered by provided?
The other possibility is to use a maven annotation processor, but last time I used it, the user experience was not great.
(also, actually, I don't think unused code could be a legal issue, but I'm no layer).

[DarrienG]

Ah I think you might be right. Let's leave it as is then. Closing this guy out.