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

fix spelling mistake
Documentation fix for the constructor javadoc

Added escaping for ampersands (&) in the constructor javadoc for the CR/LF characters, so the generated HTML does not show the literal characters, and shows the escape sequence.
GROOVY-6989: skip generated call(Object[]) method for GeneratedClosure
Merge pull request #524 from paul-bjorkstrand/patch-1

Documentation fix for the constructor javadoc
GROOVY-7122: enable typechecker passing for Iterable usage in for-in loop
Merge pull request #525 from keeganwitt/master

added/updated GMaven, GMaven 2, GMavenPlus, Gant, and Gradle info
add link between core index.adoc and groovy-contributions
 Merge branch 'contributions' of https://github.com/jnorthr/groovy-core into contributions
GROOVY-6626: Add a BigInteger.power(Biginteger) method
fix typos in groovy-contributions
Merge pull request #527 from PascalSchumacher/GROOVY-6626

GROOVY-6626: Add a BigInteger.power(Biginteger) method
update title
GROOVY-6626: Add a BigInteger.power(Biginteger) method

corrected @since
GROOVY-6618: fix several constructor delegation related vargs bugs, that where also affecting enums
GROOVY-7134: Add @SelfType annotation for traits
GROOVY-7134: Make sure "this" can be used in the context of a self type
GROOVY-7134: Add documentation for @SelfType
GROOVY-7134: Fix potential NPE and reorder tests for faster execution