Updates for Diff's readme.txt

Documentation update to the readme.txt file, and perhaps to the module page, at least the latest fixed path?

PATHS:
Main page: node/[nid]/revisions

Fixed link to latest two revisions of a given node:
node/[nid]/revisions/view/latest (directly to the "show diff" view of the latest two revisions (current and previous))

node/[nid]/revisions/latest (list two latest revisions)
Example result:
[timestamp] by [author] (current revision)
[timestamp] by [author] (previous revision)

Comparison page of the two latest revisions :
node/[nid]/revisions/view/[nid2]/[nid3]