Explain howto use maven's pre/postbuilders

They're configured like the other builders but one needed to check the
source to verify this until now.

Change-Id: I97400ad9c7cb0dfb89fe2757c4bedce639e7ea24
maven-deploy: don't require id

The id element is eg. not needed for file deployment so don't make it
mandatory.

Change-Id: Ide83b18cbab6010033c3bd8e8264595d1d2d5fa2
Merge " Explain howto use maven's pre/postbuilders"
Merge "maven-deploy: don't require id"