Add 15.7 release notes to master
Add 15.7 release notes to master

Forward port this from chef-15 branch

Signed-off-by: Tim Smith <764ef62106582a09ed09dfa0b6bff7c05fd7d1e4@chef.io>

higanworks/chef,MsysTechnologiesllc/chef,nvwls/chef,mattray/chef,MsysTechnologiesllc/chef,nvwls/chef,higanworks/chef,higanworks/chef,higanworks/chef,nvwls/chef,mattray/chef,nvwls/chef,jaymzh/chef,mwrock/chef,MsysTechnologiesllc/chef,chef/chef,chef/chef,jaymzh/chef,jaymzh/chef,tas50/chef-1,chef/chef,gene1wood/chef,nvwls/chef,jaymzh/chef,mwrock/chef,nvwls/chef,mattray/chef,tas50/chef-1,mattray/chef,mwrock/chef,MsysTechnologiesllc/chef,mattray/chef,higanworks/chef,gene1wood/chef,mwrock/chef,gene1wood/chef,chef/chef,tas50/chef-1,gene1wood/chef,tas50/chef-1