Disable annotation processor by default

[kwin]

This closes #104

[slawekjaranowski]

Should be document how to enable without - META-INF/services/javax.annotation.processing.Processor

[mcculls]

Should be document how to enable without - META-INF/services/javax.annotation.processing.Processor

Agree - another option would be to leave the services entry in place and use a system property to enable/disable (off by default)

[kwin]

use a system property to enable/disable

As this is core functionality of Javac I wouldn't introduce another toggle: https://docs.oracle.com/javase/8/docs/technotes/tools/unix/javac.html#BHCDIFEE -> option -processor <classname>. I will enrich the readme and site.

[kwin]

@mcculls Is https://github.com/eclipse/sisu.inject/blob/master/docs/index.html really manually crafted or created by some tool (if yes from what source)? WDYT about switching to a standardized Maven site here as well leveraging markdown?