Add some information about push return message format

Author: td2014

book/05-distributed-git/sections/contributing.asc

@@ -143,7 +143,9 @@ To jessica@githost:simplegit.git
    1edee6b..fbff5bc  master -> master
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-John tries to push his change up, too:
+The last line of the output above shows a useful return message from the push operation.  The basic format is `<oldref>..<newref> fromref -> toref`, where oldref means the old reference, newref means the new reference, fromref is the name of the local reference being pushed, and toref is the name of the remote reference being updated.  You'll see similar output like this below in the discussions, so having a basic idea of the meaning will help in understanding the various states of the repositories.  More details are available in the documentation for git push.
+
+Continuing with the example, John tries to push his change up, too:
 
 [source,console]
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