Update README.markdown

Added additional informations and example code for enhanced push notifications.
Merge pull request #179 from m451/patch-1

Update README.markdown
#6 : content-available should be appended to the aps node instead of the
root node
#6 : TU updated
Replacing Spring Bundler with maven-bundle-plugin
[maven-release-plugin] prepare release apns-1.0.0.Beta4