document promote_secondaries

document promote_secondaries

Write something about the tweak to enable promoting secondary addresses
instead of deleting them together with the primary address as discussed
in this thread on the netdev mailing list:
http://www.spinics.net/lists/netdev/msg52294.html

The claim that this is supported since 2.6.15 is based on looking at
changes to net/ipv4/devinet.c in the linux-2.6 git tree:

Commit to add support:
2005-05-30      Harald Welte
8f937c6099858eee15fae14009dcbd05177fa91d

Commit to fix bug:
2005-11-22      Jamal Hadi Salim
0ff60a45678e67b2547256a636fd00c1667ce4fa

Version 2.6.15 was released 2006-01-03 and seems to include the code from
both the above commits.