link to Integrated Guide added to poetry manuscripts index page

Following discussion at Camp and in consultation with Ken, I've added a link to the Integrated Guide from the Poetry Manuscripts index page, http://www.whitmanarchive.org/manuscripts/transcriptions/index.html

The purpose of the addition is to cue users to the fact that even if we have not transcribed a poetry manuscript, images of the manuscript are most likely available via the Integrated Guide.

~Liz