Added intro text + links to several Java implementations

Signed-off-by: Sebastian Hoß <mail@shoss.de>
Added reference to CDI.

Signed-off-by: Sebastian Hoß <mail@shoss.de>
Added my own DbC implementation

Signed-off-by: Sebastian Hoß <mail@shoss.de>
Added link to another license-maven-plugin

Signed-off-by: Sebastian Hoß <mail@shoss.de>
Added example for :implements

Signed-off-by: Sebastian Hoß <mail@shoss.de>