SLING-3098 - Set up code signing for the p2 artifacts

Add a 'sign' profile which activates the maven-jarsigner-plugin. Also
document the prerequisites in the top-level README.txt .

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1598130 13f79535-47bb-0310-9956-ffa450edef68