Add comments about the BUILD_NATIVE change

This is a significant change and mention of it should have gone into
UPGRADE.txt and CHANGES.
........

Merged revisions 370081 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 370082 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370083 65c4cc65-6c06-0410-ace0-fbb531ad65f3