Replaced references to wiki on contributions page with links to version control instructions

git-svn-id: https://svn.apache.org/repos/asf/servicemix/website/trunk@1566995 13f79535-47bb-0310-9956-ffa450edef68
Updated front page to better reflect what's inside

git-svn-id: https://svn.apache.org/repos/asf/servicemix/website/trunk@1566996 13f79535-47bb-0310-9956-ffa450edef68