BUILD.txt, trivial, but helpful

[Paul Baclace]

Can someone update BUILD.txt to mention "mvn package assembly:assembly 
"?  I had to dig around to find this out (again); assembly:assembly is 
not a typical maven phase and 
https://cwiki.apache.org/confluence/display/WHIRR/How+To+Release is a 
lot to look at for someone not doing an official release.

Thanks!
Paul