Added index link to the support lifecycle.

It's not clear how to find the Puppet Enterprise Support Lifecyle (e.g.
which versions of PE do we support, and to what extent). Every time
somebody needs it, they need to google it.

It could also potentially be linked to from the "getting support" page,
but it affects every release of PE is fairly important, so I put it in
the index.

Feel free to reject this and suggest a better place for it. If it's
currently linked to from another location, I haven't been able to find
it.