add groovy-contribution images
add hot-to-contribute tutorial
change 1.5.0 reference into 1.4.0

fix spelling mistake
add link between core index.adoc and groovy-contributions
 Merge branch 'contributions' of https://github.com/jnorthr/groovy-core into contributions
fix typos in groovy-contributions
update title
Merge pull request #520 from jnorthr/contributions

Contributions - How to donate documentation to groovy-core and it's components
Upgrade to Asciidoctor 1.5.0